It seems that QUIC fails to work over tailscale when using the exit node. I've installed QUIC Indicator in chrome (https://chrome.google.com/webstore/detail/quic-indicator/dghiicffobcmljfmlaedpmpmgpnjcabc) and I never see websites I know that offer QUIC/H3 using the protocol, but I do when I turn off exit node. Quick test is:

Make sure QUIC Indicator is installed
Turn on tailscale w/ exit node
Go to https://cloudflare-quic.com/
Set exit node to None
Go to https://cloudflare-quic.com/