The DM in our game homebrewed something called a Dire Otter Swarm and had the Corroded Metal attribute that can destroy nonmagical items, but he gave it a buff that allowed them to destroy Magical Items, to which the rest of our party gave him a confused look. Are we just being excessively sensitive about losing powerful items now or is that something that shouldn't have happened?
If your DM is homebrewing things, then they do whatever the DM says they do.
If that's ruining your enjoyment of the game, you should take that up with the DM in a social context. As far as the rules go, the DM makes the final call on how abilities work, and that can include degrading or destroying magical items.