A popup warns you navigating away will close your challenge, but it doesn’t actually close it.
There’s a delay, but it should after about 10-15 seconds
I watched it for >1min and it was still up before I canceled it. I just opened another challenge and closed it, it stayed up for around 20 seconds. Opened yet another, and this one remained for nearer 2 minutes.
Acknowledged, will look into it