Hello noradtux and dksoft,
I got my RB5009 this week and installed 7.1rc4 on it and decided to test PPPoE over tagged vlan, for my surprise it worked.
I do have a switch between the RB5009 and the Fiber ONT, but still tagged.
My config is below, (have we configured something differently?):
# sep/21/2021 19:50:42 by RouterOS 7.1rc4
# model = RB5009UG+S+
/interface bridge
add ingress-filtering=no name=bridge vlan-filtering=yes
/interface vlan
add interface=bridge mtu=1600 name=WAN_vlan vlan-id=35
/interface pppoe-client
add add-default-route=yes interface=WAN_vlan name=PPPoEv4 user=*****
add interface=WAN_vlan name=PPPoEv6 user=*****
/interface bridge port
add bridge=bridge ingress-filtering=no interface=ether8 pvid=88
/interface bridge vlan
add bridge=bridge tagged=bridge,ether8 vlan-ids=35
/ipv6 dhcp-client
add add-default-route=yes interface=PPPoEv6 pool-name=GUA-pool rapid-commit=\
no request=prefix use-peer-dns=no