Default water behavior is to flow to a lower elevation if possible. It only spreads evenly if there isn’t a lower elevation it can reach.

It looks like you’ve got a hole in the floor. When the water flows, it’s sorta like lightning: it opts to take the path of least resistance to get to a lower elevation. So if there is any elevation drop in a given direction, it will flow only in that direction, rather than spread evenly.

Water goes towards holes in the floor below it, so if a hole is too close, the water won’t spread. I see one behind the water next to the wall. Fill that hole and any others you find, and it should spread out.

Minecraft. Minecraft never changes.

(Except for when it does. Liquid behaviors have been less-than-perfectly-intuitive from the start though.)