Thanks for sharing the findings
It does sound like a similar event to what username2349023 is describing, where Index Recon is running to rebuild the Index Cache.
Just wondering what is the Index Cache configuration on each of the MediaAgent (involved in backing up the data). This can be found in properties of MA -> Catalog.
If you can share a screenshot of your Index that would be great. Additionally the Storage Policy (showing all the copies) and the retention.
The above will help to understand whether the Index Cache retention is long enough.