W.

Can't speak for the person who reported this of course, but for me: yes, at least a couple of times. Can't remember if this was the case in every instance it happened though.

@ANAGH Bhavsar Then that's probably the issue. That's just my 2 cents anyway; I'm quite new to Minecraft so please don't take my word as gospel. But mining being extremely slow/difficult when you're underwater in general is not a bug, but an intended feature. Efficiency on your pickaxe will help a little bit, but having aqua affinity on your helmet is more important than that if you're mining/breaking any block while your head is underwater. 

I hope you won't let losing your stuff discourage you though. Just start again, retrieve your gear if you can. And if you can't: you were able to get some nice enchanted gear, so you can do it again!

Probably a stupid question, but did you have the aqua affinity enchantment on your helmet? If not, mining speed is slowed down significantly as soon as your head is underwater. 

Not sure if this is the same issue, but since a couple of days there are NO mobs spawning anywhere near my base anymore. I haven't even seen a fish in the ocean. If I walk or swim a good distance away from my base, mobs start appearing again like normal. But nothing near my base. Even my 'pet' zombie disappeared (and yes, I had him named. Had him for a long time, and was getting quite attached to it, lol.) 

Also not sure if this is related, but since you mention being high up, I recently finished a multi-layered farm at my base. Not nearly as high as OP mentioned, my upper level is at 88.

Edit: I checked, and it seems that around 100-200 blocks in any direction from my base, mobs start to spawn normally. Since my base is on a small island and the ocean normally is full of mobs, this was easy to check.

(Am on PC, Windows 10)

Edit2: I solved the issue in my world, but still don't understand why it happened. Also, like I said, I have no idea if this is the same issue you are experiencing. But in my world, the mobs reappeared after I made a copy and killed off a lot of the animals I kept, specifically (if it matters at all) ducks and sheep. Immediately, the ocean was alive with drowned, squid, fish and dolphins again. And at nighttime the familair mobs spawned like normal. 

So for me it's solved, but I thought passive and hostile mobs had seperate mob caps? If so, this specific observation doesn't make much sense. Still, at least I can play like normal again, after I figure out in the original world how much of my animals I need to kill. I obviously don't want to have to kill them off alltogether.

Not sure if this is the same issue, but since a couple of days there are NO mobs spawning anywhere near my base anymore. I haven't even seen a fish in the ocean. If I walk or swim a good distance away from my base, mobs start appearing again like normal. But nothing near my base. Even my 'pet' zombie disappeared (and yes, I had him named. Had him for a long time, and was getting quite attached to it, lol.) 

Edit: I checked, and it seems that around 100-200 blocks in any direction from my base, mobs start to spawn normally. Since my base is on a small island and the ocean normally is full of mobs, this was easy to check.

(Am on PC, Windows 10)

Edit2: I solved the issue in my world, but still don't understand why it happened. Also, like I said, I have no idea if this is the same issue you are experiencing. But in my world, the mobs reappeared after I made a copy and killed off a lot of the animals I kept, specifically (if it matters at all) ducks and sheep. Immediately, the ocean was alive with drowned, squid, fish and dolphins again. And at nighttime the familair mobs spawned like normal. 

So for me it's solved, but I thought passive and hostile mobs had seperate mob caps? If so, this specific observation doesn't make much sense. Still, at least I can play like normal again, after I figure out in the original world how much of my animals I need to kill. I obviously don't want to have to kill them off alltogether.

Same issue on PC. Been happening since before the recent updates - for me at least. Like OP said, destroying and replacing the torch makes the area light up properly again. It happens only very occasionally for me though, but it does happen in every world.