I was testing and found out that the swOS 2.4 does not read a QinQ Service VLAN tag.
Setting the port as VLAN Mode Enabled and VLAN Receive Only Tagged does not work with Service Vlans, had to set the Vlan Receive to any for it to work.
Tested with a CSS326-24G-2S+RM, setting up the vlan tunneling with a CRS125-24G-1S-IN and a CRS326-24G-2S+RM, both with routerOS.
Also noticed the swOS 2.3 does not work with QinQ vlans at all, not even setting the port as Vlan Receive any.
Is this the expected behavior on the CSS software?