Relevant configuration below.
Yes, I know there will be comments about the problem being elsewhere and the whole configuration is required but I’ll take my chances. Things work, so this is more a case of leaving some information in case others encounter the same problem.
/interface bridge
add admin-mac=XX:XX:XX:XX:XX:XX auto-mac=no dhcp-snooping=yes frame-types=admit-only-vlan-tagged name=bridge priority=0x1000 vlan-filtering=yes
/interface bridge port
add bridge=bridge frame-types=admit-only-untagged-and-priority-tagged interface=ether1 pvid=32 trusted=yes
add bridge=bridge frame-types=admit-only-untagged-and-priority-tagged interface=ether2 pvid=31 trusted=yes
add bridge=bridge interface=ether3
add bridge=bridge frame-types=admit-only-untagged-and-priority-tagged interface=ether4
add bridge=bridge interface=ether5
/interface bridge vlan
add bridge=bridge tagged=bridge untagged=ether3,ether4,ether5 vlan-ids=1
add bridge=bridge tagged=bridge,ether3,ether5 vlan-ids=101
add bridge=bridge tagged=bridge untagged=ether2 vlan-ids=31
add bridge=bridge tagged=bridge untagged=ether1 vlan-ids=32