The problem occurs with the same apps you use in your demo — 1Password and Files. I will chalk this up to the continued extreme bugginess / instability I’m observing in iPadOS 15 and Shortcuts.
In fact, while troubleshooting this problem I removed Files and left the single Open App action for 1Password. First time I ran it from the dock with another app open it worked as expected. Subsequent uses failed — it simply opened 1Password in the main screen (1Password was not a Slideover app at that point). If I edit the Shortcut to remove the Open App - 1Password and then re-add it, it works one time.
This is one of many problems I’m observing (and many have noted) with Shortcuts. Virtually unusable and a complete exercise in frustration. At least in my experience.