I can try that, but to be clear they are both using the same IoT VLAN.
All data from the old DECO AP gets the IoT VLAN tag by the switch.
All the data from the IoT SSID from the U6 Pro gets the IoT VLAN tag.
The only difference is where the tagging occurs… With the working configuration, the Deco is leaving it untagged, and then switch is tagging it. With the non-working configuration, the U6 Pro is tagging the messages.
I don’t really think it’s a VLAN issue. If it was some firewall issue or similar, then I would expect it to have the same issue on both. Because the messages are receiving the IoT VLAN tag before being routed.