Might be beating a dead horse again here, but:
Remote location with MX68, a couple of AP's, central DHCP, multiple SSID's with tagged VLans, all VLans + Native in the trunk ports for the AP's. All VLans setup to relay to same central DHCP, central DHCP setup with subnets for all VLans.
DHCP works for all VLans except one, removing the tag in that SSID gives IP in the Native VLan, as expected I believe.
In my head things are configured correctly, but before I setup packet capturing, maybe someone can point out the rotten egg - thanks!