Occasionally when playing, I will see a block, or several blocks, that are inexplicably invisible, showing the sky behind them (even though I'm looking down) and either outlines of far-away mobs, or entire underground structures that I haven't explored.
Example image attached.
This example uses seed: -413209067 in creative mode.
I'm flying around, observing the area. A large amount of area in various biomes has already been flown over.
At time time I captured this screenshot, my character could not move, it got stuck at the coordinates shown in the image. The only way I could escape was to teleport somewhere else. Usually, though, when I fly or walk over the "invisible" surface blocks, the entire scenery disappears until I fly out of the boundaries of that "invisible" region. When walking, I could stand right on the edge and look down, but as soon as I stepped over the boundary, all scenery disappeared.
During a game session, if this occurs, it's repeatable. You can re-visit the invisible area. However, restarting the game, I cannot repeat it. I've observed this multiple times, in different games and seeds.
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I think this is a duplicate of MCPE-19308.

It could be related to MCPE-19308 although that description is about render speed, and I have no problem with that. It's also reported for a different OS; this is Windows 10.
My render speed is fine, and my render distance is set to the maximum 16 chunks. My son and I play in the same world; he on an iPad mini and me on Windows 10. We've only seen this on the Windows side. But apparently others are seeing it on different platforms.
There have been similar incidents, NOT when flying around, but when digging a tunnel in close quarters. I'll encounter a block that looks invisible (I see bright sky beyond even though I'm not looking up), and allows me to see mobs in the distance as silhouettes. I don't see underground structures though that invisible block though. Digging out and replacing the block fixes the problem. Hmm, next time I see the issue for which I took a screenshot, I'll try walking up to the edge and digging an invisible block to see what happens. I haven't tried that yet.

I also experience this on Windows 10, so I know what you're describing. MCPE-19308 doesn't just talk about slow rendering, it also talks about slow chunk generation. (Generation is creating the data structure in memory, either by loading it from the save file or by building it from scratch if it's new. Rendering is creating the screen image from the in-memory data.)
Slow generation is what you're experiencing. It occurs when you're exploring a large area quickly, usually while flying. That requires loading a lot of chunks and generating new ones, and these are relatively slow operations. If you go so fast that the game can't load or create them fast enough, it may lose track of some chunks it's trying to load or generate. Either way, those chunks can't be rendered, so you see through them, often all the way to the sky on the "other side" of the world. (Entities always appear, because they're rendered separately from chunks. They look like silhouettes when they're in a dark place. Some block surfaces in chunks on the other side of the "hole" also render, similar to Java's Spectator mode if you're familiar with that.)
Slow rendering causes the same appearance, but it usually corrects itself after a short time or when the player moves.
The slow generation and slow rendering appearances are similar. They may or may not be caused by the same bug, but that's up to the devs to figure out, so currently we're tracking them as the same issue and why I said I think this is duplicate of that. A moderator will review and correct me if I'm wrong.

Thanks so much for the explanation. Slow generation would explain the picture I attached, causing some surface chunks to be lost and therefore not rendered. Maybe I was flying really fast when that happened, but after it happened, I could leisurely walk along the edge of the hole.
On the other hand, slow generation wouldn't explain why I occasionally see an 'invisible' block while digging underground. I'm not moving fast, I'm moving at the rate of 1 block every second or so. In this case I don't see underground structure through the 'window' but I do see silhouettes of mobs in the distance. It's almost as if the game loses track of a single block instead of a whole chunk.
The last time this occurred, I had just started a Survival game (no flying). I didn't explore, I just harvested wood from a nearby tree and immediately began digging a temporary shelter using a pickaxe from the starting bonus chest. I had dug a stairway down about 4 blocks and was digging out a small room for a crafting table and a furnace, when one of the blocks on the wall became invisible, showing bright blue sky and a distant silhouette of a skeleton bobbing up and down, likely in the river near where I spawned. I wish I'd taken a screenshot.
I've also seen a floor block do this. The first time that happened, seeing bright sky through a hole in the floor, I thought I had broken through the bottom of the world, which should be impossible in survival mode (and I hadn't dug anywhere deep enough either). I'm not convinced this has anything to do with slow generation or slow rendering, because I'm barely moving. And when it happens underground, it typically affects just 1 block.

You're right, the single invisible block is an unrelated issue. I remember reading about it, but then forgot while reading about the slow generation and rendering. Since the latter is already on the other ticket, could you please simplify the description to describe just this issue? Something like what you wrote in your last comment about it would be fine. (I actually saw this once in my own world, I think, but chalked it up to a temporary glitch. It hasn't repeated.)

Let's just link this as duplicating MCPE-19308 and close this ticket as a duplicate. I've written a new clean ticket MCPE-30451 for the invisible-block-while-digging issue.

Very good, thanks for being so patient and cooperative!
Mods, please close as a duplicate of MCPE-19308 per OP's comment above.