TUF-AX3000 strange behavior on WAN / Internet Connection page (both UI types)

[encip]

Router Model Affected Models: TUF-AX3000

Firmware Version Affected 388.2 - both traditional and TUF

Is this bug present in upstream Merlin releases too? No

Describe the bug I have noticed various odd problems with the WAN / Internet Connection settings page. After submitting the page, it seems like it will accept the settings such as WAN Connection Type and Username/Password, but they will be hidden/reset when you return to the page. The current IP will also show as 0.0.0.0. Sometimes the Apply button at the bottom of the page does nothing- it changes highlight on mouseover but does nothing when you click.

To Reproduce Steps to reproduce the behavior:

1. Factory reset
2. Go to WAN / Internet Connection
3. Change the WAN Connection Type from anything other than the default, say PPPoE
4. Enter a Username and Password
5. Click Apply
6. Return to the page and observe that the WAN Connection Type will have reverted and the Username / Password will be missing

Expected behavior I expect the WAN / Internet Connection page to update to the newly saved settings but it does not. However, the values do appear to be saved somewhere in the background, as the PPPoE still logs in successfully.

[encip]

The WAN Index / WAN Type dropdown is new and I don't recall seeing it before. Notice in the screenshot that the dropdown is empty. Is this option copied from firmware of a different model? Does the router believe it's in a Dual/multi WAN mode of some sort even though Dual WAN is turned off?