MagicDNS interferes with non-tailscale traffic on cellular WAN (DNS64)

**Describe the bug**
When tailscale is running with MagicDNS enabled, and when the client laptop is hotspotted to a phone with cellular WAN, non-tailscale traffic is interrupted (presumably because of DNS resolution issues...?).

**To Reproduce**
Steps to reproduce the behavior:
1. Hotspot your phone and tether your laptop to it.
2. If tailscale has MagicDNS enabled, and if it's running, you will intermittently be unable to access non-tailscale network resources.
3. `sudo tailscale down` and `sudo systemctl stop tailscaled.service` restore access to other resources.
4. Disabling MagicDNS (but keeping tailscale running) also restores other network access.

**Expected behavior**
tailscale shouldn't interfere with access to other network resources. For now I've disabled MagicDNS, but it's super handy, so I'd love to be able to use it!

**Screenshots**
Just let me know if a video would be helpful -- happy to share.

**Version information:**
- Laptop: Ubuntu 20.04
- tailscale: tailscale commit: 64a9656c01754b6652994cb3a8ef59bce1246cfc, 1.4.4
- Cell phone: Pixel 5
- Provider: Google Fi

**Additional context**
A similar behavior _occasionally_ occurs when I switch WiFi networks, but generally this isn't a problem with wireline WAN.