Realtek RTL8326 and RTL8326S haw flawed IEEE 802.1Q VLAN support.

Namely - chip can’t insert 802.1Q tag into frames, however all other aspects works correctly.

Technically, 802.1Q-enabled chip must support 4 modes of frame transport - see below:

Ingress port Egress port Proposed action RTL8326 action
Untagged Untagged Forward frame intact Forward frame intact
Untagged Tagged Add 802.1Q tag Forward frame intact
Tagged Untagged Strip 802.1Q tag Strip 802.1Q tag
Tagged Tagged Forward frame intact Forward frame intact

This bug renders 802.1Q support in RTL8326(S) almost useless. There is no fix or work-around, sorry.