Thanks for the update. Yes, i also agree something may have changed. Reading the release notes for 9.7.1 at PDF page 6
https://support.ruckuswireless.com/do...
There is a mention of "5.1.5 - Resolved an issue with APs being unable to join ZoneDirector due to the default MTU size set on Zone Director running on version 9.6 . (ID ER-929)"
this is my guesswork, you may not have seen this issue in 9.6 however there was something related in 9.6 which got fixed in 9.7.1 however now you may be seeing the manifestation of incorrect fix. again this is my guess nothing for sure.