I had spent my last prisms and saw I was able to grab the two from the Heavy Metal track. Grabbed them thinking the bug was fixed, but I needed more to MW some armor. Went to Rahool and grabbed the three cheap ones for cores, so I should have five. Went to MW my gear and it says I have no prisms. The three from Rahool are just gone. For whatever reason, the three prisms I got from Rahool are somehow different, as I was able to get those (even though they never went to my inventory) and the regular exchange says not enough space. The ones from the Heavy Metal reward track had a black background and aren't even considered prisms by the game. Very weird. So, I deleted the two from the track and I can again get prisms from Rahool. I also cashed in my remaining coins from the event for cores and did not get any. Very frustrating.

Same issue here as well. I am not maxed out on "Enhancement Prism" in my inventory since I am able to collect them from other events when available. Not being able to collect the "Enhancement Prism" is only occurring within the "Heavy Metal" activity. Video link showing details. https://youtu.be/3hDMC4P4kGQ

It's indeed bugged and if you actually claim it, that will lock you out from collecting more prisms.

It's not just on the event card that you can't collect Prisms. If I have even 1 prism in my inventory, I can't collect prisms from any vendor or my postmaster. Correction: It's the Heavy Metal event card prisms that are glitched. If you empty out your prisms to collect the ones in the Heavy Metal event card, you won't be able to pick up prisms from other sources.

I came here for this problem. I have 92 and it says i have too many to collect.