This repository provides an easy way to reproduce issue on haproxy 2.4.3 with not working websockets when the backend has enabled communication over http/2.
docker-compose up --build
then check https://localhost:8443/test.html to observe failure and https://localhost:8444/test.html that should print timestamps in random delays as expected. Also https://localhost:9443/test.html is exposed to show that without haproxy everything works as intended.