Hey, this has been an issue since updating from 11.4, I'e checked various versions and it it's the same for all.

I suspect it may be because I have an icon pack added, but wasn't an issue prior.

I only use Player / Target but seems to be happening to both.
No LUA Error for this.

Update: Seems to be an interaction with Masque, when disabling it to run on BigDebuffs, it still picks up the square icon.
Installed Cirque - Simple Skin which has a default circle icon, seems to look ok for now.

Issue resolved with most recent update.