Thanks for reaching out to us for your concerns.
Can you please let me know the exact version number of Adobe Acrobat DC that your are working on ?
I Tried this at my end and Drag drop in Page Thumbnails panel in a pdf / across pdfs is working fine for me.
Also i would request you to kindly share with me a screen recording video of the steps that you are trying to do here ?
You can send me that at firstname.lastname@example.org
Ayush JainGary Brazzell commented
Just developed this problem today, out of the blue. Zooming out does not help. It simply won't let me rearrange pages. The main way I use PDF is to create a file by combining different files. In the first screen, I can usually drag and drop thumbnails. Now I can't drag thumbnails at all, not from any of three panels where I would usually do this. Also, my tools are all messed up. The main tool I ever use is the zoom / magnification, and it's gone.