There is so much to like about Affinity apps, but it also makes oversights like these even more apparent.
By default, users cannot move an Asset from one subcategory to another in the Assets panel.
However, the UI reacts in a way that makes the viewer believe that they can move an Asset. When the user begins to drag the Asset, a drop cursor appears in the destination subcategory. This UI response should not happen if the user is not allowed to move the item to begin with. No error or explanation is shown.
After Googling it, I discovered an Affinity forum post from 2016 by staff moderator @MEB:
This solved my issue. But it’s been nearly 6 years since this original post, and this is still the default behavior in Affinity Designer 1.10.4. And the online documentation doesn’t even mention that this is a required keyboard shortcut. And though the Assets help page does cover what subcategories are, it doesn’t even bother to explain that one needs to use a shortcut in order to move items in between subcategories.
There really has got to be a better way. Please, at least update the online help so that people aren’t stuck having to search Google for a solution. At the very least, this should be mentioned in your online help. (Edit: The keyboard shortcut is mentioned, but only in the context of manually re-ordering assets, nothing about moving assets between subcategories)