When upgrading fcv to 5.0, the configsvr will patch-up the sharding metadata. In particular, it will set a 'timestamp' on the config.databases documents.

If this patch up happens concurrently with a commitMovePrimary, it could happen that commitMovePrimary reads the current config.databases entry (without timestamp), then the setFCV thread patches up the entry so it has a timestamp, and finally, the commitMovePrimary writes the updates database version without a timestamp.