As per upstream:
After a HelloRetryRequest has been sent, the client may negotiate a lower protocol that TLS 1.3, resulting in an invalid state transition in the TLS State Machine. If the client gets into this state, incoming Application Data records will be ignored.
Upstream bug: https://bugzilla.mozilla.org/show_bug.cgi?id=CVE-2019-17023 (currently private)
This flaw causes the client to hang when there is a downgrade attempt. Therefore no actual protocol downgrade occurs.