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 email@example.com
Ayush JainClif commented
I started a new job and we have the full version of Adobe here. I use this function daily and have this same issue. I was playing around trying to figure out why I can't rearrange or move a page from one PDF file to another and I noticed this setting. I am not sure if this is related/the cause of the issue but when I went to preferences -> Security -> Document Restrictions Summary shows "Document Assembly: Not Allowed". This document isn't secured but my software that I am using (and then printing as adobe pdf) might automatically do this. Could this be setting be the cause of my issue?