UTM, HA - FATAL: could not receive data from WAL stream: FATAL: requested WAL segment 00000001000007B1000000FB has already been removed

I noticed today that we seem to have issues on our Sophos UTM 9.510-5 system in HA mode. We are getting continuous errors like this:

sophosutm-2 postgres[18415]: [4-1] FATAL: could not receive data from WAL stream: FATAL: requested WAL segment 00000001000007B1000000FB has already been removed

and

sophosutm-1 postgres[14369]: [3-1] FATAL: requested WAL segment 00000001000007B1000000FB has already been removed

They occur every 5 seconds, so there are literally 1000's of these lines in the logs.

sophosutm-2 postgres[21561]: [3-1] LOG: streaming replication successfully connected to primary

Note: the reported WAL segment is always the same 00000001000007B1000000FB.

The system seems to be running fine, otherwise, but these messages are making us feel uneasy.

Ideas?