Hi Christian,
Thanks for highlighting your issue with Linuxbridge driver and VPNaaS.
I would like to ask you to consider the following, which will not give you an answer or solution to your problem but perhaps helps to understand and accept if any specific issue has not received the needed attention from the community.
Basically it is the core team (and some enthusiasts around it) who do bug triaging in Neutron, they are experienced and willing to provide answers, solutions.
The possible combinations of drivers, extensions, backends, deployment tools is huge, and there are combinations which are not well covered in the current community, like linuxbridge is not that used (OVS and perhaps OVN are the top tested and deployed, at least from latest user surveys). So it can happen that in the current core team nobody has the special knowledge and tools (test environment, hardware...) to debug a specific issue.
Neutron maintains a list of "lieutenants", to make easier to contact the right person:
For VPNaaS the people are not active anymore in the community.
In such difficult to debug situation it is really helpful for the community if you can test the issue on current master code (it is possible that the issue happens only on older branches) and with tools that are available for most of us, like simple devstack.
Regards
Lajos Katona (lajoskatona)