While playing on Minecraft 21w37a, I have noticed a similar bug related to this one. At first I wanted to report that bug, but then it may end up being a duplicate of this one. All liquids (ie. both water and lava) seem to have completely stopped flowing, regardless of whever they were placed from buckets or generated by the world generator. When a new world is created, the generated water and lava do not flow either. Placing and breaking blocks next to them does not seem to be making them flow either.
I agree, that could indeed be the core issue. If the debug menu were to only show/hide when the F3 key is pressed rather than released, then this bug likely wouldn't be a thing since Minecraft does not allow you to show/hide the debug menu while the game is paused.
Sorry about the previous comment that I posted and deleted, that was a mistake. I mistook the other issue that I thought I duplicated.
I was using a vanilla client at the time. So there were no mods or anything like that. A few minutes ago, I attempted to reproduce the bug once again, but couldn't do it.
I apologize about that. The video got deleted when I removed the channel that the video was uploaded to. I have recorded new footage, uploaded it, and updated the link. I have also updated the bug description and the steps to reproduce the bug to make it easier for everyone to reproduce the bug themselves.
I just tried reproducing the bug in 1.19.4. I coudln't reproduce it. It is possible this issue got resolved.