Latest 1.7.10 version
When right clicking lava with an empty cell you get 8 lava cells
one lava cell = one bucket of lava

Could you tell me where is the issue so I can fix it myself

Humm yeah thats an issue, I may not fix this for 1.7 though

I cannot seem to find it having after having a quick look.

If you want to fix it, edit that and bump the version and I can push it

Seems that this is the cause of problem
https://github.com/TechReborn/TechReborn/blob/1.7.10/src/main/java/techreborn/compat/recipes/RecipesIC2.java#L463

oh yeah, good spot. Even a github web edit should fix it.