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One possible cause for this is that there may be numerous archive jobs for different agents, and from the same core, running to the same storage location/path.
One, or some of the jobs may have the following recycle action selected:
Replace This Core
This will delete an pre-exising data pertaining to this core, but leave data for other cores intact.
Even if one of the jobs has this action selected, all other archives from that core will be deleted, and the next time the scheduled incremental archive for those agents runs, it will need to do a full archive again firstly.
There are a number of possible solutions: