First I would try to execute the MAPI call:
curl -ik -H "Authorization: HCP XXXXXXX:YYYYYY" -H "Accept: application/xml" "https://CLUSTERNAME:9090/mapi/services/replication/links?prettyprint"
to see how many links show up.
Then pick one of the links that DOESN'T appear in HCM and see if you can get its statistics via MAPI:
curl -ik -H "Authorization: HCP XXXXXXX:YYYYYY" -H "Accept: application/xml" "https://CLUSTERNAME:9090/mapi/services/replication/links/LINKNAME?verbose=true&prettyprint"
if you get statistics from MAPI but not from HCM then I suggest you open a support ticket w/GSC.