Hi, sorry for the delay.
I talked to my colleague, who is more knowledgeable on the subject, about this. It seems you are just unlucky with your motherboard. Apparently, some BIOSes just change the pci numbering on reboot. In a future patch there should be a feature that warns you when a different pci devices is passed through than the time before but it isn't possible to automatically find the right device, since there is no way to identify unique devices (unlike NICs, pci don't have something like a mac address, so it wouldn't be possible to tell e.g. two gtx 1050s apart from one another).
What you can do is see if there is a new BIOS for your mainboard that fixes this. If it doesn't you could also write a little script that, on boot, checks lspci, parses out the ID you need and then overwrite your vm .conf. (not ideal, I know, sorry). Or, not reboot too often and manually check.
Hope this helps
