I'm 99% sure the ONT will bridge VLAN 101 to the ethernet port, not all of the PTM interface. That's how the HG612/ECI modems work and I'm not sure why you weren't doing that on the Draytek as AFAIK its how everyone here utilises bridge mode.
The only occasion I can think of why they might not have done that is for people early on where voice may have gone over its own VLAN and so it would need to pass that to the router, although I think those people the telephone was handled in the ONT anyway so probably still only passed VLAN 101 traffic untagged out the ethernet port.
There is no logical reason to pass the entire PTM interface over to the router when it only needs VLAN 101.