Hello James,
I looked into release notes of both 9.6 & 9.7 and could not find anything relevent to your issue.
It may be a corner case and would strongly advice that you upgrade to latest stable code after following right upgrade path (refer to release notes & consult support team )
Look at memory consuming features and whichever not relavant & not actively being used - disable them. Keep any eye on guest passes creation.
Hope this helps.