If you want to help test a workaround (disables TLS 1.3) for this issue see: https://github.com/torproject/tor/pull/625
Nick wrote (https://trac.torproject.org/projects/tor/ticket/28973#comment:2 ):
I expect that a few warnings will still happen with this branch: it waits for the bug to happen once before disabling TLS 1.3, by which point other TLS 1.3 connections may already be in progress.