Pool is unable to write txns after two nodes adding

Build Info:
indy-node 1.2.50 (rc)

Steps to Reproduce:
1. Install pool of 4 nodes.
2. Run load tests to fill ledger with ~9000 txns.
3. Add the 5th node.
4. Send NYM txn.
5. Add the 6th node.
6. Send NYM.
7. Get NYM.

Actual Results:
Both nodes catches up successfully but NYM from Step 6 is not added (but GET_NYM command from Step 7 still works).

Expected Results:
Pool should work normally after both nodes adding.

Workaround:
Restart at least 4 nodes of 6 to return the pool to valid state.

Additional Info:
The issue is not reproducing with small ledger pool.