Not every timebut it happened to me several times:

1. In Google Pixel's stock Photos gallery app, select 25 pictures (might reproduce with less, not sure).
2. Share to Commons app, complete the Upload Wizard, submit.
3. Because this is share intent, you are sent back to the gallery app.
4. Do something else with your phone for 20 minutes, like reading emails, opening other apps, but do not open the Commons app.
5. After this interlude, open the Commons app.
6. Problem: Zero picture have been uploaded. Upload seems paused. See screencast below.

I am 100% sure I never tapped "pause" since the last time I reinstalled the app 7 days ago. Yesterday I successfully uploaded a 40-pictures set via the same gallery app's share button, but I had opened the Commons app shortly after launching the upload.

Workaround: Tap the "unpause" button. Upload starts smoothly.