This one seems unsolvable unfortunately and just seems to be some weird issue with Forge, as the LavaFluid class itself looks for fluids within the fluid tag "Water" which Liroth Fluid and Flowing Liroth Fluid are in, yet the result is not what was to be expected, for now I shall label this as unsolvable and if I hear of any update, I'll reopen the issue, for now, I shall sadly close this one.

I've seen this issue before in other mods, so I know there's a way to fix it. You may want to reach out to some people who do underground biomes like the Volcanic Caverns guy or someone from the major biome mods.

I don't remember closing the issue after, I may have hit an extra button, but you are right that I do need to contact another mod author, I just need to contact one who has experience with modded fluids.