I always used (and still want to use) PS Express from within the Apple Photos app.
So when I‘m in Edit mode in the Photos app I used to tap on the … button to get the list of Photos plugins and chose PS Express from that little popup window. This immediately brought me to PS Express where I could do my edits and then smoothly return to the Photos app.
This used to work on my iPhones for a long time. And it still does work on my latest iPad Pro 12.9“ (2018, 3rd gen).
However, on my new iPhone XS Max (iOS 12.1) this doesn‘t work anymore. Instead, when trying to invoke PS Express in the above described way I get an info screen from PS Express that tells me I have to use the share sheet way of getting my photo into PS Express and edit it there.
Here‘s a little screencast I made that shows the problem: