I fixed it by renaming all the crafting categories in 248k (yes, that was quite a lot of work), since the FNEI code isn't nice to work with - though I think that replacing the "_" with eg. "~" in lines 69, 80 and 89 as well as replacing "[^,]+" with "[^~]+" in line 117 in FNEI/scripts/settings/elements/crafting_buildings.lua should also resolve the issue.