Why is AfO brought to the foreground when one of its actions is the last block in a workflow?
If you just came here for a quick workaround: Go to the app's Settings tab, and configure your preferred "End of Workflow" there. (This feature was added in v1.4.0 for iOS.)
Actions for Obsidian becoming the focussed app like this on iOS is an unfortunate side effect of Actions for Obsidian translating between Obsidian and Shortcuts, and the way the Obsidian API is set up (no blame intended!) — and as of the time of this writing, a satisfactory solution eludes me.
If you want to know the background and learn about the struggle (it's real!), continue to Why do the AfO actions behave differently than most other apps' actions? 😉