This sounds very much like the problem that Tartiflette ran into with the Unknown Skies mod:
. Unfortunately, he never revealed exactly what was going wrong, but after poking through the mod itself I've got a theory...
In \data\campaign\procgen\planet_gen_data.csv, it looks like he replaced the "category" of some of his custom planet types:
#US_lava,PLANET,cat_lava,10,-100,-2,2,,-1,around_giant_at_any_offset,0,,,100,150,220 220 220,255 255 255,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,
#US_volcanic,PLANET,cat_lava,10,-2,0,2,,-1,around_giant_at_any_offset,0,,,100,150,220 220 220,255 255 255,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,
US_lava,PLANET,cat_toxic,10,-100,-2,2,,-1,around_giant_at_any_offset,0,,,100,150,220 220 220,255 255 255,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,
US_volcanic,PLANET,cat_toxic,10,-2,0,2,,-1,around_giant_at_any_offset,0,,,100,150,220 220 220,255 255 255,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,
My theory is that using the "cat_lava" category is what caused his custom US_lava/US_volcanic planets to generate without any planetary conditions. If that's true, then it's theoretically possible that other categories (say, "cat_cryovolcanic" for example) might cause a similar problem when used for custom planets...