Strangely, after completing the rebuild the array as reported in IMSC appears to be normal and back down to just two drives. Peculiar.
It may seem strange, but what you saw was the expected behavior. You started out with 2 drives in a RAID1 configuration. Let's call them drives A and B. Then Drive B failed. You removed Drive B, and inserted Drive C, and flagged it for a rebuild. When you removed drive B, you effectively had a RAID 1 volume with a missing disk that was rebuilding to a new disk C. The RAID controller will not "forget" about drive B until it has fully rebuilt to drive C. Only once drive C is fully operational will the RAID controller consider drive B to be replaced.