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
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It's resolved as "Works as Intended."
If you disable the vanilla definitions of fluids and fail to provide your own, liquids will lose behavior.
Haha no worries, it can be weird behavior for sure.
In case you didn't know, fluid behaviors come from the water.json and lava.json fluid tags in the vanilla data pack. If you provide those files in another (enabled) data pack, fluids will work as you expect.
It's the same for all vanilla tags. Leaves will decay because their tag is missing, but you can provide the tag in your own data pack.
Hey, tryashtar? Not to sound salty, but I just tried to explain this issue isn't resolved yet, or prevails under different circumstances. I've got an updated screenshot to prove it a-la 1.13