I was able to reproduce this, at least in part.
If the entire page is not visible in the right-hand pane, dragging and dropping a thumbnail out does not work *unless* multiple pages are selected.
Either (making the Acrobat window larger) or (changing the zoom) such that the entire page fits in the right pane allows dragging and dropping the thumbnails to work again.
This seems like a bug, yeah?
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It's not a bug. When you're zoomed in the zoom box appears in the page panel and if you drag and move it around you'll change the current location on the page. To select the page object itself you need to click outside that box, or zoom out so that the entire page is visible, as you've found. It's not very convenient, but also not a bug.
The old "it's not a bug, it's a feature!" has been communicated to the user. Thanks.
Take a look here for an explanation about how you need to drag a thumbnail when not the whole page is displayed (the article is about duplicating pages, but the same "dragging" rules apply):