FTS: Server groups incorrect index partitions distribution after groups modification

Build: 7.0.1-5916

Test scenario is the following:

Initially I have 2 server groups:

sg1

172.23.100.11 - kv

172.23.100.13 - fts

sg2

172.23.100.12 - kv

172.23.100.14 - fts

FTS index has 1 replica and 6 partitions

So, expected initial index distribution should be like this:

1 replica per server group - this is happened

6 partitions per server group - this is happened.

Then I'm adding another fts node to sg1, perform rebalance. So, resulting cluster configuration is:

sg1

172.23.100.11 - kv

172.23.100.13 - fts

172.23.100.19 - fts

sg2

172.23.100.12 - kv

172.23.100.14 - fts

Expected index distribution is:

1 replica per server group - this was not happened

6 partitions per server group - this was not happened.

Resulting partitions distribution is:

sg1:

172.23.100.13

"default:fts_idx:num_pindexes_actual": 4,
"default:fts_idx:num_pindexes_target": 4,

172.23.100.19 

"default:fts_idx:num_pindexes_actual": 4,
"default:fts_idx:num_pindexes_target": 4,

 sg2:

172.23.100.14

"default:fts_idx:num_pindexes_actual": 4,
"default:fts_idx:num_pindexes_target": 4,