Here is a video of the bug sent to me by another user on Wiktionary, I can also record one myself, if that helps.

Same here on English Wiktionary (iPhone 15 and MacOS)

Adding onto Izno's comments, After the administrators of EN wikipedia purged the cache of the respective pages my attempts to clear the app cache and delete and reinstall the app did not change anything the images were still there. When clicking the images though, they either 1) didn't appear, or 2) brought up a different photo. The reason why I think it's a bug (though I could be wrong) is that it doesn't make sense for the header to show one image and then bring up a completely different one once clicked. It's a bit odd that the headers in the App aren't cleared with cache or when the page is purged by an administrator, or even after a week the image is removed (which probably shouldn't happen). Perhaps this is just vandalism but imo it's a bit of an oversight to allow the header in the mobile app to show an image that is not visible on the website, because it allows hidden vandalism.

Yes admins in the discord were trying to resolve the issue by looking for vandalized templates and purging the templates and the pages that use them. But the images are still appearing in the app. Which is why I was asked to bring up the issue here.