Wither Despawning After Initial Explosion

Multiple times, my boyfriend and myself have attempted to summon in the Wither into our realm. The Wither will spawn in initially, do their load up explosion and then completely despawn and is no where to be found. After many annoying attempts by resetting the server, we attempted to copy our Realm into a regular world and attempt to summon the Wither there to see if it would work. It did not. We attempted to summon the Wither at bedrock level, and surface level, neither of which worked. We cleared blocks below and around the soulsand T formation in attempt to see if this was the issue, still did not work. Any and all attempts at fixing the issue has not worked. It feels like the only option is to go into Creative mode, give myself a Wither Star and return back to survival, which is not preferable as we do not want to lose achievements.