Warbringer Negative Armour Break Interacts Strangely With Minions

If you have a minion that breaks Armour - they will usually only break armour to 0. Then if you hit it, the armour tooltip seems to behave strangely. It starts at negative armour and slowly goes "up?"

For example it starts at -15k, and then goes up to 15k over time. Very confusing.