If using "read the entire restore point from source backup" option, note that active full backup method does not always copy the most recent restore point from the source backup repository. If the necessary recent restore point has not been created by the time the GFS task must start, Veeam Backup & Replication will copy the latest available restore point from the source backup repository.
So it should not be related to the length of the source jobs` chains.
I've had this running for a few months and in one example, on 8/30 a monthly was created. Then my next monthly is dated 10/14? One of my copy jobs ran all of September but failed on the last day of the month. It continued on the 1st of oct but the first monthly was on 10/7? Its like it deleted all the points from Sept.
What was the duration of those monthly runs? If data transfer does not fit the synchronization interval, the synchronization interval is extended with the mentioned option.