umqtt (with tls): check_msg() / wait_msg() - how to know connection is gone? timeout?

I'm using the umqtt module - on conjunction with ussl_mbedtls - within the UNIX port with mbedtls.
When I re-route the traffic originally going to my MQTT broker to /dev/null (e.g. 127.0.0.1), check_msg() as well as wait_msg() continue behaving as before. No different return value, no exception thrown.
From the umqtt.robust implementation I figure that should be the case though.
I also thought this might be due to an overly long timeout, so I called settimeout(X) on the underlying socket, but then wait_msg() throws an exception right after X seconds - no matter what - which I also appears weird to me.

What's the way to go to catch network issues and potentially reconnect automatically when using umqtt.{simple,robust} with TLS?

Start by reading the umqtt.simple and umqtt.robust implementations, focusing on check_msg(), wait_msg(), and their handling of the underlying socket. Reproduce the behavior on the UNIX port with ussl_mbedtls, including settimeout(X), and compare the observed results with the expected connection-loss and reconnect behavior described in the issue.