We're facing the similar issue with Brand Portal integration. The client secret keeps on dropping and we noticed that the older bundles installed are the culprit. If you have done in-place upgrade from older versions then their might be unwanted SP/CFP bundles.
You can safely uninstall those bundles. We were on AEM 6.2 (CFP15) and we recently migrated to AEM 6.5.5 and facing the similar issue and the culprit was some bundles installed during CFP 10 on AEM 6.2.
If you are not using, uninstall ACS Proxy bundle and see if that helps.
Let me know.
Thanks,
Singaiah