When it jumps back to your applications' root view controller, is the preview interact-able at that point? If not, then the preview is showing a snapshot of the content when it first came up, and likely the app has crashed.
It would help us to take a look at the diagnostics when this happens. When you reproduce the problem in Xcode:
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Either (a) an error banner will appear, click the "Diagnostics" button in that banner. Or (b) if you’re not seeing an error but you still want to provide diagnostics, you can get the same diagnostics window by going under the Editor menu in the menu bar, then selecting the Canvas submenu, then selecting “Diagnostics”.
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In the sheet that appears, click "Generate Report" in the bottom left of the sheet
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Attach (or make from the folder) the resulting zip file to the bug (will be named something like previews-diagnostics-0123456789.zip)