Some Menyoo DLC vehicles do not have available modifications apart from colours even in the shop.
I spawned the Gauntlet Hellfire and when I tried to edit it, I noticed it only had colour and a few other insignificant secondary options to change. Suspension, engine, splitters, spoilers etc where not displayed on the mod menu or LS Customs. I tried another (Krieger) and the same thing happened. However it didn't happen to the Pariah (on which it happened months before and went away on its own). What could have messed with the car modification files?