[Bug] New app's thumbnail is not loaded if image is replaced inline (without changing its URL)

This issue initially was opened as https://github.com/nextcloud/server/issues/34151 and has been moved here. It may be related to https://github.com/nextcloud/appstore/issues/891 and https://github.com/nextcloud/appstore/issues/361.

Bug description

I have this line in my app's appinfo/info.xml:

<screenshot small-thumbnail="https://raw.githubusercontent.com/nursoda/twofactor_email/main/screenshots/select-auth_thumb.png">https://raw.githubusercontent.com/nursoda/twofactor_email/main/screenshots/challenge.png</screenshot>

I replaced that screenshot image on GitHub two weeks ago. In the app store, the new screenshots and thumbnails are displayed:

Not so in /settings/apps App's Details – there, the old image remains:

Clearing cache and reloading does not solve it.

Steps to reproduce

1. Create a NC app, do screenshots, link them with a static URL in info.xml, publish the app in the NC app store. Install the app in a NC instance. Check that images are shown in app store and NC instance.
2. Change screenshots and upload them to the SAME static URL. Check the app's details page in the store: new images are shown. Check in the NC instance: old image remains.

Expected behavior

The new thumbnails should be displayed in /settings/apps aswell.

No source files or tests are named in the issue. Start by reproducing the stale-thumbnail case through the app's proxy and trace how image URLs are fetched and cached; done means a screenshot replaced at the same URL is refreshed in /settings/apps without requiring a URL change.