PPPoE with VLAN not coming up?

[neillans]

I'm seeing an odd issue with PPPoE connections using a VLAN where they don't connect properly ... but when I go and start a tcpdump on the interface everything is connected on to capture the handshake etc, things connect.  Initially I thought this was just coincidence, but I've reproduced this multiple times.  Connections on the same interface work fine without VLANs, so it's specifically when VLANs are added to the mix.

Hardware is a PC Engines apu2, running OPNSense 20.7.4.

ISP has configuration which allows multiple PPPoE sessions on different VLANs (to which they then bind different IP's to for public access), so we have a session on the "main" interface (igb2) for a "shared" IP, and then a single vlan at present setup which doesn't connect until a tcpdump on igb2 is started.  This occurs whether the main "shared" connection is enabled or not so that seems irrelevant. 

Happy to provide configuration files and logs if required, please just specify what you need. Also happy to do any testing needed, as it's easily reproducible here!

Tested with a MicroTik router separately to verify that it wasn't anything else on the network or at the ISP end.