So this is still prevalent in Minecraft, but the devs have said this has been fixed. I feel like nobody has been able to properly explain the issue or perhaps this is a completely different one. Disabling the vanilla datapack causes fluids to no longer work as intended.
lava uses the water texture and both fluids behave like flowing air.

The tricky thing no one has reported properly about; this only occurs when you leave the world and come back, as that fully refreshes the world and datapacks. My speculation is there's something fundamentally wrong with how Minecraft handles re-loading the vanilla datapack. (perhaps fluids aren't tagged as 'essential'?)

To reproduce this issue:
1. Use the command /datapack disable vanilla
2. Save and Quit to Title
3. Rejoin the world
You must save and quit, simply using /reload will not work