When an Oracle of Battle selects the "Skill at Arms" mystery they become proficient in all martial arms and heavy armor.
When a +3 set of mithral Full Plate is made the character is no longer proficient, but is proficient with +3 full plate.
Additional issues discovered, even with being marked as proficient penalties are being applied as if non-prof. But adding the heavy armor prof feat makes it work
Is there a reason why we can't grant the heavy armor prof via the mystery? instead of trying to fake it with "AUTO:ARMORPROF|ARMORTYPE=Heavy"
There is nothing fake going on. Mithral changes the proficiency required to Medium. I can fix the present issue easily enough.
Which is why the feat was altered to this:
I was getting armor check penalties when equipping the +3 fullplate too though. Which didn't happen if the char has the Heavy Armor feat