I'd speculate that the problem isn't related to the VPN connection, but to how the Mullvad client app handles the interface (especially if keepalives frequently

I'd speculate that the problem isn't related to the VPN connection, but to how the Mullvad client app handles the interface (especially if keepalives frequently fail and the interface keeps getting downed). Even the official clean OpenVPN app has its own sense of up and down tunnels, which could cause similar problems. Wireguard, at least in its official linux implementation, is much lighter weight and if necessary reestablishes an active session on demand (whenever there's traffic) without any interface-level tweaks. That's probably invisible to applications, or at least much less visible even if there are low-level syscalls that can inspect the state of a wg tunnel interface.