All Machine-Type monsters on the field gain 200 ATK. Each time a face-up Machine-Type monster(s) on the field is destroyed by battle or card effect, place 2 Junk Counters on this card. You can send this card to the Graveyard, then target 1 Machine-Type monster in your Graveyard whose Level is less than or equal to the number of Junk Counters that were on this card; Special Summon that target.
Read the card. By saying "monster(s)" it means that if multiple are destroyed at the same time you only get one. Union monsters don't add counters if they're destroyed while equipped, because they are treated as spells while equipped. No bugs involved, it's just not as good as you thought it would be.