The Metal Carapace impulse says:
Sheets of bent and rusted metal cover you in an armored shell. The carapace is medium armor but uses your highest armor proficiency.
So, even when not trained in medium armor, it should still use your highest proficiency. I have a character spec’ed out that is trained in Light armor but the proficiency bonus is not being applied.
Should have an AC of 10+2 (dex) + 3 (light armor proficiency) + 3 (item) but instead has 10+2+0+3.