Sorry for the late reply, unfortunately this wasn’t a priority after Christmas. Anyways, in the meantime I have been doing some networking testing with docker, which doesn’t support SR-IOV by default so I had to rely on some scripts (https://github.com/jpetazzo/pipework) to move the VFs in the containers’ namespaces, and I found the following (un-)interesting fact which ended up (apparently) solving the issue also for LXD.
Basically, the NIC of the PF that I use for experiments is called eno2, and when I add new virtual functions some end up being named ethX and others eno2vX, but not always in the same way. Well, the ones named ethX always ended up causing all sort of issues because of the naming, so I wrote a script to rename them as eno2vX, so that the names of the NICs of all VFs would follow this pattern. And after that, all the problems mentioned above were gone. I didn’t go into the details, but if you need more info/details which might help (or you happen to know the reason behind that behavior as I am no networking expert), please let me know 