We have now tested https://github.com/kamailio/kamailio/tree/grumvalski/disable_http2_multiplex... and it seems to solve our problems!
@miconda I would ague that this is a better solution than disabling TLS ALPN. Now we can continue use http2, which is a more efficient protocol than http 1.1, but just disable the multiplexing, until we have the full refactoring in place.