I am using Adobe Acrobat Pro DC 2019.021.20061
When I open a PDF and double-click with my middle mouse button (the scroll wheel), a mini page thumbnail window is opened, similar too the Pan & Zoom window, but slightly different. For a picture of what I mean with the mini page thumbnail see this post: https://superuser.com/questions/428987/how-to-disable-middle-button-showing-a-page-thumbnail-in-adobe-reader

Note that this is not regarding the Page Thumbnail sidebar nor the "Pan & Zoom" window. There is no menu option to make this mini page thumbnail appear, the only way is to double-click the middle mouse button.

While that mini page thumbnail window is opened, deleting the currently visible page that contains any visible form fields makes a dialog appear that reads "One or more pages are in use and could not be deleted"

When the page deletion is done using JavaScript, it doesn't matter what the currently visible page is, the operation will thrown the following error in the JavaScript Debugger: "RaiseError: One or more pages are in use and could not be deleted."

After the initial failed deletion that produces the error, it is subsequently possible to delete the page without issue.