Adding a node when master is failover (No Rebalance), complains Rebalance In Progress

1. Create 4 buckets
2. Add 2 nodes to the cluster
rebalance params :

{'password': 'password', 'ejectedNodes': '', 'user': 'Administrator', 'knownNodes': u'ns_1@172.23.109.179,ns_1@172.23.109.172,ns_1@172.23.109.180'}

3. Load data to all the 4 buckets
4. Check cluster for number of nodes
current nodes : [u'ns_1@172.23.109.179', u'ns_1@172.23.109.172', u'ns_1@172.23.109.180']
5. Find the master node in the cluster
/diag/eval status on 172.23.109.172:8091: True content: 'ns_1@172.23.109.172' command: mb_master:master_node().
6. Failover the master node
fail_over node ns_1@172.23.109.172 successful
7. Now add a node to the cluster
adding remote node @172.23.109.181:8091 to this cluster @172.23.109.172:8091
2019-11-15 22:04:32 | ERROR | MainProcess | test_thread | [rest_client._http_request] POST http://172.23.109.172:8091/controller/addNode body: password=password&hostname=http%3A%2F%2F172.23.109.181%3A8091&user=Administrator headers:

{'Content-Type': 'application/x-www-form-urlencoded', 'Accept': '*/*', 'Authorization': 'Basic QWRtaW5pc3RyYXRvcjpwYXNzd29yZA==\n'}

error: 400 reason: unknown ["Node addition is disallowed while rebalance is in progress"] auth: Administrator:password

Behavior is different from build 6.5.0-4783

*Update*

This issue is first observed on build 6.5.0-4219.