Steps to reproduce:

1. Find/Make a very flat area
2. Place one water source
3. Go to the corner and build like I did on the images below

Observed results:

Part of the water source you placed "uselessly" will somehow flow up on a weird way

Expected results:

The water physics should still make sense

How I discovered this:

I tried speedrunning the game, while for almost 6 years I didn't even kill the Ender Dragon. When making the portal without mining obsidian(a speedrun tactic), but next to naturally generated flowing water, I consider this really weird. Anyway I was playing on 1.17.1 except on 1.18 because the world is less deep to find lava.