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Apologies if this has been reported before.
I tested this out, and I'm unable to do properly drag-and-drop on an account with more Collections than will fit in the menu without scrolling (via Chrome). I can drag-and-drop it within my field of view, but anything outside that it doesn't scroll with me to see where I'm dropping it. Assuming this is part of the issue.
Assigning to conceptDawg, with the caveat that it's also getting deferred because this may get fixed in the LT2 redesign.
I added a new collection just now and wanted to move it to the bottom of the list, but when I clicked-and-dragged, it would move but not stay where I put it. I noticed that I can successfully rearrange collections only when I'm scrolled to the top of my catalog page. If I'm mid-page, nothing stays moved in the collections list. In the first screenshot, look at the catalog scroll bar on the far right. It's at the top, and I can move the collections. In the second screenshot, the catalog scroll bar is a bit down from the top and I can't rearrange the collections. I'm using Windows 10/Chrome 78.0.3904.87. It does the same thing in Internet Explorer, not that anyone should be using IE these days, but I'm trying to work in two accounts at the same time...
(If I knew how to make gifs, I'd totally have animated example screenshots right now.)