When I broke an adjacent block next to a lava source block. the lava didn't update and didn't flow, the lava just stood still and didn't flow. I tested this multiple times and the same thing happened. When I mined around the source I noticed it was only 1 source block exposed next to a mineshaft, which may be a factor. I tried searching if others have reported this bug but could only find related bug reports, nothing such like this.
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When I replaced the lava source with a block and broke the block there was no air block underneath. Also if it helps narrow down the bug, the seed of the word is 5887315100460707616 and the coordinates of the lava source block is 193 11 87. It may be slightly off as the block was already replaced and I'm having to recall it by memory as I forgot to turn on debug screen in the screenshot, but the general area is for certain correct and should at most be a few blocks off. Also also, I just entered the world seed and came to the location, and found out the lava flows fine, But in my initial world, this wasn't the case, a key difference is in the initial world the chunk was rendered from 1.16.5, but in this new world with the same seed in 1.17 prerelease 4, the chunk the lava was in is rendered in 1.17 prerelease 4, perhaps that is another factor.
Ok, yeah, so this happens because of the mineshaft.
Fluids in Minecraft work this way: They flow straight towards the next lowest point, in this case there was only one way for the lava to go – straight into the mineshaft.
Now, if you mine the top block first, the lava will still see the mineshaft as lower and flow into there instead of into your mining operation. Then if you mine the block below it, the lava won't get updated and you get the situation shown in this screenshot where I've excavated a bit. Since the lava wasn't updated it didn't realize it can flow down towards the camera too:
[media]If you do it the opposite way, it'll behave as if you'd expect.
Fluids in Minecraft are quirky. I'm not sure if we have another bug report for this but it's not a bug, it's how fluids are supposed to behave, even if it seems a bit odd.
Is there perhaps an air block below it that the lava is flowing into?