I’ll just reiterate an important footnote about the federation issue that was encountered with mastodon.social, as I’ve added inline, specifically:
[1] The issue was later identified as a combination of: some behavior that’s common to anyone using Fastly (as mastodon.social uses), TLSv1.3 support, and a bug specific to a version Erlang OTP regarding TLSv1.3 middlebox compatibility mode.