Replica is not turned into active when server is added back into cluster

I start with a cluster of 2 servers. Create a bucket of type couchbase and set it to have 1 replica.
Then I upload 100 K documents to the cluster. The data gets divided across the servers perfectly.
First server has 50K active and 49.9K replica and the second server has 49.9K active and 50K replica.

Now I remove the second server from the cluster and rebalance. Now the first server has 100K active. This is still perfect. Now I add the second server back into the cluster and rebalance. Now the first server has 50K active and 49.9K replica but the second server has 0 active and 0 replica.

Why isn't the 49.9K replica from the first server replicated over to the second server as active and the 50K active from the first server replicated over to the second server as replica like it did the first time?