If I recall correctly, heavy survivor gear covers you so that your butt (if it was an included body part) and just above that would be covered. Kinda hard to fit in a tail when there’s so little space.
Check to see where exactly it covers and check the description as well. It probably needs the modifier ‘can accommodate mutated anatomy’ (or whatever that’s as labeled) to fit.
Although this begs the question, did you mutate your tail shields in the armor? Because if you previously were able to put it on while already having your tail then that’s a bug. If you mutated while wearing it and it’s not compatible and nothing happened to the armor then that is either a bug or an oversight.