Also having this issue. I've been using private bot matches to learn how the different heroes actually feel in real gameplay (I've been finding it much more informative than just playing in the sandbox) but the insistence by the servers that I finish them when I'm the only one even playing and was just trying to teach myself the different heroes is really, really annoying. The ban also prevents joining more private bot matches, which is how I found out that this was even a thing.
I must say, the first time I saw the message about being unable to play I was extremely baffled and also somewhat scared that I'd get banned from the playtest or something because of a bug when I've not even played a single round with other human players yet.
