LASTPROCESSEDVERSION (a.b.v. LPV) action for CDCR breaks down due to poorly initialised and maintained buffer log for either source or target cluster core nodes.
If buffer is enabled for cores of either source or target cluster, it return -1, irrespective of number of entries in tlog read by the leader node of each shard of respective collection of respective cluster. Once disabled, it starts telling us the correct LPV for each core.
Due to the same flawed behavior, Update Log Synchroniser may doesn't work properly as expected, i.e. provides incorrect seek to the non-leader nodes to advance at. I am not sure whether this is an intended behavior for sync but it surely doesn't feel right.