Maybe I'm not understanding properly, but why can't my device ARP ping and handshake with the subnet router to determine that I'm on the local subnet and to stop routing it through Tailscale?
Tailscale intentionally overrides your device's routing table to force traffic between hosts in the same subnet to go over a Wireguard tunnel instead of bypassing it. They do this because they believe that the presumption that a local subnet is trustworthy is false.