Smart Group relative date criteria not saving
I recall we've experienced this for quite a number of fields in Advanced Search. Some of them are - Event Dates, Registration Dates and some date filters in Case section.

I did a patch to fix this for us but missed to update core for some reason. If the above fields fail for you, can you try applying this patch and see if it works for you?

Opened a JIRA for this issue with a PR https://github.com/civicrm/civicrm-core/pull/11486.