I might need to mention, that the VLAN 9 is in the Device br-guest, and that there is a separate Wifi with separate subnet assigned to the Interface GUEST as of now.
If the interface associated with VLAN1 is set as eth1 (as compared to eth1.1), your experience makes sense. When an interface is setup without the ".x", it is treating that as untagged. If it has the ".x" it will look for a tagged network.
The br-lan bridge device is setup with port "eth1", therefore the CPU should be untagged for VLAN1.
If, on the other hand, it said:
then it would expect the CPU to be tagged. You can try it yourself -- change the bridge to eth1.1 and change the switch configuration to tagged on the CPU -- it will continue to work. But if you do either one individually, it will fail.
Changing them in LuCI may be tricky (it may require a few extra steps), but you could change it directly in the file (or using UCI commands). However, there is no real reason to do so unless you have a specific purpose for the change.