HTTP/2 stream head-of-line blocking

Your mobile API moved all client traffic onto a single multiplexed HTTP/2 connection per client (over TLS/TCP) to cut handshakes. It works great on good networks. But users on lossy mobile networks now report that when ANY request stalls, ALL their in-flight requests stall together and then recover together, even small independent calls. Server-side everything is healthy and fast; the server's per-stream metrics show no slow handlers. Packet-capture from affected clients shows TCP retransmissions right before each multi-stream stall. How do you triage and what are the options?