Can't set baby jumbo frames on PPPoE connection

[Super-Hands]

Router Model Affected
DSL-AX82U

Firmware Version Affected
3004.388.8_2-gnuton1

Is this bug present in upstream Merlin releases too?
No

Describe the bug
Unable to set MTU on PPPoE connection to 1500 - an error comes up and says it needs to be a max of 1492 even with jumbo frames enabled in switch config

To Reproduce
Steps to reproduce the behavior:
Go to Wan pppoe settings and try setting 1500

Expected behavior
Should be able to set a mtu of 1500 on the pppoe connection (which should also set mtu of 1508 on the wan interface. This was fixed in upstream merlin, changelog for that is:

FIXED: PPPoE with an MTU of 1500 requires the WAN interface
to have its MTU set at 1508 (patch by pinwing)