QinQ is confusing inside RouterOS.
The “user service tag” should be named “Set Ethertype = 0x88a8 (= 802.1ad Service Vlan)”
“user service tag” is confusing, because we can mix with “customer service tag”.
To clarify : “service tag” is the outer tag (provider vlan or “vman”), “customer tag” is the inner tag for the final client vlan tag.
More, 0x88a8 is a predraft value. The standardized value is 0x9100. We should be able to choose 0x9100 value as well.