In most cases you have A Regular Player report a bug from his modded game.
There is no way to tell what version, variant or configuration of the mod he's using or what "improvements" he may have made to it even if the name of the mod is stated.
Tracking a "bug" like that is a complete waste of time and testers have work to do.
OTOH: Modders have reported many valid bugs because they do understand what's going on - even if the bug only surfaces in the very specific conditions of their mod.
That's why we tell people to report that in the mod's topic. If a modder finds a bug that would also occur in the vanilla game then that's a wholly different category.