Issue with cover graphic reload

Hello
I have an issue with the cover graphic and Sweetdate Theme. When I upload or delete a cover graphic, changes are visible only after a reload but not immediately. This is how I can replicate this issue with the original Sweetdate Theme – and not with my child version.

I click on “Change Cover Image” and then on “Select your File”
I get the feedback “For better results, make sure to upload an image that is larger than 940px wide, and 225px tall.” but the image is NOT displayed.
When you reload the browser you see the image.

Now when you click on “Delete My Cover Image” I get the message
“Your cover image was deleted successfully.” but the image is still visible.
Just when you reload the cover image disappears.

When I do the same on the rtMedia Test site all works fine and immediately.

Any ideas?

Thanks
Andreas