I had Possibility Storm on the field along with Storm of Saruman and Thousand Year Storm. I cast Brainstorm (this was later in the turn so I had already cost other spells.) It was exiled by Possibility Storm like it should but it wasn't copied by Thousand Year Storm (like other spells are normally, even with Possibility Storm on the board.) The real issue is that when it was exiled Possibility Storm then went through cards and somehow found the same Brainstorm that was exiled (which shouldn't be in the deck) and then returned back "you can't cast this spell".

On PC. (Windows.)

Brainstorm should have been exiled and Possibility Storm should have found any other instant from my deck besides Brainstorm since it shouldn't be in the deck to be found.

I have no idea what is going on here but was in the middle of the match so I only grabbed a screenshot after I forfeited.