If you disable the vanilla datapack, the lava texture turns into the water texture. It doesn't matter if you place lava, then disable the datapack, or vise versa. 

 Tested on 1.13.1 and 1.13.2-pre2

https://gyazo.com/ae70a8128c682b3551e96311d3e5239a

How to reproduce:

1. Place lava.
2. Disable vanilla datapack.
3. If lava texture does not change right away, place water next to it.