When I activate the timer it goes back to my main screen after a while, but the timer stays active. I can change faces, check different apps, etc, and when I check the timer again it’s still counting down.
it seems to have failed a couple of times when I’ve used a preset (if memory serves, and it might not), but I don’t use the presets anymore anyway, I just use Siri, tell it what to set for, and it keeps counting down no matter what other things are going on.
Try not using presets, see if that helps.