If the two drives were EVER assigned to the same pool, even briefly, they "permanently" remember that status because BTRFS keeps a record of pool members totally separate of Unraid's pool definition. To remove them from the BTRFS pool can require extra steps, @JorgeB is the resident expert on what is required. Theoretically if the drive is removed from Unraid's pool correctly, the status is updated and things work as intended. If you physically remove the drive, I believe it may still have the BTRFS headers on it that make it a member of the pool, I think you have to leave the drive installed and remove it from Unraid's pool definition, then the pool is rebalanced to remove it.
I'm a little fuzzy on the exact procedure, but what you experienced is a symptom of both drives being assigned to a single pool at some point in the past, and not being properly BTRFS balanced to separate drives.