Most switch configurations will have a default native vlan of 1 unless otherwise set. There may also be an option to disallow untagged traffic on a trunk port, but based on your description I don't believe this to be the case.
It sounds as though the current IP address associated with the Instant is valid for the default native VLAN. When you change that mgmt vlan on the IAP side, is the IP address still valid for the new vlan?