Is anyone experiencing issues of flowing water becoming source blocks?

These source water blocks all came from some flowing water from the top of the staircase.

I'm playing the Fabulously Optimized modpack (version 6.4.0-alpha.2).

Edit: this bug is even weirder than I thought. The water flow and source blocks change based on where I am. In the following video, as I go down the stairs, the water spreads. I make a hole to breathe, and as I go away, the hole gets filled in. When I go back to the hole, the source blocks expand and I need to dig up to breathe. https://youtu.be/PLsbvyL0rH4

Edit 2: annoyingly, I wasn't able to reproduce this issue in a new world in 1.21.3 Vanilla or Fabulously Optimized.