Thanks for your feedback! It shows that this new addition is not as self-explanatory as I hoped and indicates features or ways of using that are either not intended or not implemented.
I already had a similar (slightly more controversial) conversation regarding this via PM yesterday, which showed that this feature is likely to cause confusion or to disappoint expectations. I've originally simply intended it to be a more detailed placeholder image for covers of files with varying covers. As a bonus, it showed the dropped covers that would be added when saving.
I know that I can't meet every expectation. But I also don't want to introduce features that require extra explanation, especially for experienced users like you, or that only end up highlighting what isn't possible. So, I'm reverting this change with Mp3tag v3.29f.