Looking at that “traceroute” output, both computers are doing what I would expect. That is, they are forwarding the packet to the default route, which should be your VPN router. In one case, that router is then further forwarding to the other computer and in the other case it isn’t.
It seems to me that you need to look at what the router is doing.
Possibly one of the computer is broadcasting route information which the router is picking up. I’m not sure why that would change after 2 weeks. Or maybe the router is generating routes from traffic that it is seeing on its interfaces.