Hello Simon,
To better understand what you are experiencing, please provide us with more details such as:
 
- AP/Controller model 
- Firmware version 
- Are you using a Virtual or WING Exrpess controller? 
- Steps taken to make VLAN setting changes
I ran a quick test on one of our WiNG Express APs and was not able to reproduce the issue.  
Kind regards,
Chris
					
				
			
			
				
	Christoph S.