During a split (A, B, C, D) -> (A), (B, C, D) -> (A), (B), (C, D) -> (A, C, D), (B)
There was topology update with BCD in CH, but stable topology is still ABCD.
When a merge happens, the merged topology has only CD as newMembers when computing the new availability mode. Therefore, the ACD does not become available though it has enough data from the stable topology.