OP-1 only registers 3 keys at a time on right bank?

I recently picked up a used OP-1. I noticed that on the right side of the keyboard, I can only hold 3 notes. When I press a 4th one, it cuts out the 3 notes and there’s no sound until I release 1 of the 4 keys (so there’s 3 or less keys being held).

I have POLY voicing on and I tested it in the debug mode to verify this behaviour. Is this normal?