websocat sometimes doesn't flush pending data, hangs

I'm running:

$ websocat \
        --exit-on-eof \
        --binary \
        tcp-l:127.0.0.1:1234 \
        wss://1.2.3.4:1234/url

When I send a large packet to 127.0.0.1:1234, sometimes websocat doesn't send it to the destination immediately (and just sits there until something causes it to send the pending data).

If the destination sends a PING, websocat does flush pending data.

If I add --ping-interval to websocat's command line, that also causes websocat to send pending data when it sends PINGs.

I can reproduce it with v1.12.0 and current master (34ddb4d1416887059fb249cc97de1538598fed9c).

I can only reproduce it when the target is a remote host and not localhost (probably because the buffer needs to actually pile up locally for the problem to happen).