Sorry you're having problems!
What device are you using Photoshop Mix on, please?
Happens on iPhone X and iPad Pro 10.5 all with latest updates.
Sent from my iPad, so please forgive my fingers' typos.
Hmmn. I'd be tempted to uninstall/reinstall the app, and try again.
I've just tested on my iPad Pro, and it works fine, to Camera Roll, and Lightroom.
Make sure that your projects are all synced before you delete the app.
Hope it works out for you, but if not, come back.