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 JainMelissa commented
This problem just started for me! What a nightmare! What should I do? Is there a solution for this issue?