While investigating the interaction between ECN and RTO, I noticed that FBSD sends out retransmissions on ECN-enabled flows with the ECT0 codepoint in the IP header - which is only permissible with ECN++ (work in progress as IETF draft). This is not a critical deviation from the specs, only a lack of conformance.
https://reviews.freebsd.org/D23118 Please review and comment
Fixed in base r357115.