I open an image in Designer (see Screenshot file with horse head icon) , then I create via Photo Persona a pattern, back to Designer and then the pattern falls outside the canvas too.

[Bob-NL]

[walt.farrell]

Welcome to the Serif Affinity forums.

I don't think your workflow is clear from that description. You mention starting in Designer, then using the Photo Persona to create your pattern.

The problem with that description is that Designer does not have a Photo Persona. 

Do you mean that you used File > Edit in Photo to transfer your document to the Photo application, and then did something and then transferred back from Photo to the Designer application by using Edit in again? 

Or something else?

[loukash]

2 hours ago, Bob-NL said:

the pattern falls outside the canvas

No "bug" here. This is by design:
Viewed in Designer, the Photo's "canvas" is actually a page. Like in other design or layout applications, outside the page you can then place objects that you don't want to use but you may want to see them while working.

Since a pattern layer is literally an infinite object, it will logically appear all over the place.

To clip the page/canvas like in Photo, select View > View Mode > Clip To Canvas.