Direct connection failure between home peers despite no restrictive NAT

Tailscale netcheck reports MappingVariesByDestIP=false (indicating non-restrictive NAT) on home network, yet some peers still route through DERP relay instead of establishing direct connections. One PC on same home network connects directly while another does not, despite firewall being disabled.

Run 'tailscale netcheck' on both home PCs to compare NAT traversal results. Check Windows Firewall status on the PC that fails direct connection, even if it appears disabled. Verify that UDP ports 41641 are not blocked at the router level. Compare network adapter settings between the working and non-working home PCs. Test if creating explicit allow rules in router firewall (even when disabled) resolves the issue.