Lost-Men Brutes don't Freeze if mob and shield were hit at the same time with Freezing Shards

Lost-Men Brutes and their variants (Stoicsmith, Lunabeard, Filthskull, etc) will have a bug where their Freeze Build Up goes to 100%, doesn't freeze the mob, and then the icon never shows again.

I played around with this since it was pretty random if they would freeze or not. I went into a group of 5 Brutes, hit them a couple times with Freezing Shards (with Chain and Pierce gems) and then spammed Frostbolt until their Freeze Build Up hit 100%. None of them frozen and the icon never showed up again.

I then ran around and spammed Ice Nova at any other Brutes I saw. They would always freeze.

I also had one instance where I was fighting a Filthskull, froze it with just Ice Nova, and then hit it with Freezing Shards/Frostbolt and not be able to freeze again.

I then tried using Freezing Shards with different gem setups:
-No gems: The bug would trigger if I hit the mob at an angle behind it so 1 shard hits the mob and another shard hits the shield. Otherwise it would freeze if I just hit in the back (hitting just the shield doesn't apply build up, but that's expected).
-Chain: Would not freeze half the time. Most likely because the first shot hits the shield and then chains to a different mob.
-Pierce: Would never freeze if I hit the shield from the front and the pierced shot then hits the mob.

It seems like hitting the shield and mob at the same time with projectiles are causing the shields freeze immunity to be permanently transferred to the mob.