This is still an issue. I've tried everything listed here and still not able to submit the app.
Once I upload a screenshot from Appstore connect, I get the popup saying "We'll use your 6.5" display app previews and screenshots..." and I see the images uploaded in the background. But after I upload all the screenshots and Submit Review, I get the message saying "There are still screenshot uploads in progress".
Here are the things I've tried with no luck -
I've uploaded screenshots one by one
I've uploaded screenshots in bulk
I've uploaded screenshots and given it a lot of time
Tried uploading screenshots by clicking "View All Sizes in Media Manager"
Tried removing screenshots one by one and re-uploading
Tried removing screenshots by clicking "Delete all" and re-uploading
Tried different browsers - Chrome, Firefox, Safari
Tried uploading, waiting and refreshing (still shows grey upload images in Chrome)
Checked the network and noticed
POST https://appstoreconnect.apple.com/iris/v1/appScreenshots returns a 201 and
PATCH https://appstoreconnect.apple.com/iris/v1/appScreenshotSets/<GUID>/relationships/appScreenshots returns a 204
Technically no errors but still not working