The reason for that is in earlier versions than 3.5, AP firmware version automatically used to change with the vSZ/SZ upgrade. However, the concept of Multi-Zone was introduced in 3.5 and above, which slightly changed the upgrade workflow where the system and the AP zones upgraded independently.
For this you may have to restore to the original 3.2 cluster backup and then upgrade to the following path 220.127.116.112 --> 18.104.22.168.217--> 22.214.171.124.296 --> 126.96.36.199.227 --> 5.x
If you have AP zones that are using 3.2.x and the AP models that belong to these zones support AP firmware 3.4 (and later), change the AP firmware of these zones to 3.4 (or later) to force these APs to upgrade their firmware. After you verify that all of the APs have been upgraded to AP firmware 3.4 (or later), proceed with upgrading the controller software to release 3.6.x and Later.
We strongly recommend to check the Release Notes of each version on supported path and supported AP platforms before you proceed with upgrade.https://support.ruckuswireless.com/documents?filter=83#documents