MediaTek folks also got back to me in a private message, confirming
the issue and also clarifying that the length of the tag is the
limiting factor. Every 4-byte tag can work, sizes other than 4 bytes
cannot. As MediaTek's tag format includes the 802.1Q VLAN as part of
the tag itself I suspect VLAN offloading will still need some extra
care to work on non-MTK 4-byte tags (like RealTek 4B, for example)...

My suggestion is to enable it only when sure (mediatek tag) and drop
it otherwise. Something like this:

https://github.com/openwrt/openwrt/commit/2603d6d81d1a088c6ad271fe20a63fa6e3a75124

It is not worth it to risk enabling offload for unknown cases.

As Realtek was mentioned, the 4-byte tag (rtl4_a) is for old SoCs,
hardly paired with a mediatek SoC. The newer tag, rtl8_4, is already
too big (8 bytes).

Regards,

Luiz