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.