Here are the steps on iOS to fix it:
If the shopify iOS app was getting stuck launching external apps like Joovii. Solution is:
1. On your phone go to settings > Apps > Shopify.
2. In the options there tick the one that says “Remove all sessions on next launch”.
3. Then close the Shopify app, and restart it. It will require you to login, but after that it is able to launch apps successfully without the session getting stuck.