Unable to delete photos transferred with Move to iOS

I cannot delete old photos that were transferred from my Android phone using the Move to iOS app. The delete button is greyed out/missing for these photos.

This is what I have observed so far:

• Then I select multiple photos, the delete button is greyed out
• When I view these old photos individually, I only see "share" and "info" buttons, but no delete or like option
• For newer photos taken on this iPhone, I can see the delete buttons normally
• These photos are NOT synced to iCloud: I checked iCloud.com and only found about 200 photos there, none of which are the ones I want to delete
• The photos only appear in the Timeline view, not in any Albums or the Recents album
• The photos don't appear in the "From My Mac" album or any similar synced album
• Under Settings > General > iPhone Storage > Photos, I see 66.94GB of "Documents and Data" but no mention of synced photos from a computer
• My Nextcloud Photos app also cannot find these photos
• The "On My iPhone > Move to iOS" folder in the Files app is empty, and there's no DCIM folder visible

I already tried to

• Restart my iPhone
• Reset Location & Privacy settings
• delete single photos instead of bulk selection
• Look for the photos in different albums and the Files app

Is there any way of deleting my photos?