Mod_proxy_http2 - got a 503

Hello all

We are trying to use the mod_proxy_http2 of httpd. Our use case is for now
- multiple backedns (configured in HTTP2)
- one httpd acting as proxy
-- mod_proxy_http2 is activated
-- we use the Balancer directive to load balanced accross backends
- the client is a curl -kv --http2 ...

The tests are the following:
- while using curl to access one of the backend directly, the access log
shows that we use HTTP/2
- then the balancer directive is configured with <a href="http://mybackend1" title="http://mybackend1">http://mybackend1</a>
In taht case, while accessing to the proxy with curl, we see that the
communication between curl & httpd is HTTP/2 but that the communication
with the backends is HTTP1/1
- then we change the balancer directive to h2://mybackend....
In that case, the curl returns 503 (like if the proxy is unable to route).

I suspect a misconfiguration but any help is welcomed. Does someone already
did something similar?