None of the feats from the Armor Master's Handbook seem to acknowledge heavy armor proficiency if not all multiclassed classes gain heavy armor proficiency.
For instance, Bulette Charge Style has heavy armor proficiency as a prerequisite. I have 9 levels fighter (proficient in heavy armor) and 1 level barbarian (not proficient). Bulette Charge Style shows "profwitharmor: TYPE.Heavy at least 1" in red. This error persists even if I add the armor proficiency (heavy) feat, oddly enough. The program only allows adding the feat if I delete the barbarian level or turn on the Armored Hulk barbarian archetype that grants heavy armor proficiency. Feats from other sources (such as Fortified Armor Training from Ultimate Combat) do see the heavy armor proficiency in the same set of circumstances.
Another issue regarding feats from this source is the "Advanced Armor Training" feat on a fighter, which unlocks a class feature option "Advanced Armor Training Option", one of which is "Armor Specialization". Armor Specialization unlocks a class feature category called "Armor Specialization Choice" but which only contains Heavy Armor, Light Armor, and Medium Armor Specializations. Technically these choices should be specific types of armor (full plate, breastplate, leather armor, chain shirt, etc.) and not weight categories. When attempting to add any of the three specializations offered, I get an error: "Failed to add Ability due to null. See log for further details." Log text attached.