Is there a reason for inconsistency in what a VPN breaks and doesn't break?

Questions
2

Random IP changes when browser cookies don’t identify with the previous activity will do that. Plus, VPN creates an encrypted tunnel so if you unless you use Tailscale + VPN only, it should work. If not and if you keep changing your set up, your issues are common and inevitable as far as I know.

But there could be other reasons for the inconsistency. Tech is always changing. Perhaps someone else will have a better and a more conclusive answer for you.