How can I stop Clock face app reverting to system on Apple Watch?

I’ve downloaded a new watch face via a 3rd party app, but every time the watch screen reawakens it reverts to the previously chosen system clock face. The new face only stays on whilst I have the third party app running, but this doesnt last more than a few minutes. I’ve tried the return to clock settings but nothing works. Is there a solution to this?

[Re-Titled by Moderator]

Original Title: Stop clock face app reverting to system