after extracting and trying a few things with debug prints, it looks like FNEI is managing to get an empty recipe in the list for raw-borax and niobium-ore, associated with the borax-mine and niobium-mine respectively.
when clicking through for niobium, it sees
tailings-borax-niobium, in buildings
imposter-minable:niobium, in niobium-mine, with a well formed recipe
niobium-ore, in niobium-mine, with a recipe that prints as { "__self": , }
This is admittedly probably better routed to either the FNEI fellow or pyanodon
pycoalprocessing/prototypes/recipes/coal-processing-recipes.lua adds recipes for raw-borax and niobium-ore, with their categories connecting them to the borax-mine and niobium-mine. pycoalprocessing has both a recipe-category and a resource-category for both "borax" and "niobium", and FNEI doesn't handle that well
the extra recipes are the upgraded ones, raw-borax is 150 syngas for 5, vs 50 syngas for a 70% chance of 3 on the minable
niobium is 70 syngas and the drill-head for 12 on the recipe, 60 syngas for 1 on the minable