Sounds like it was down to user rights, since it is now working, I would not disturb it
however Win 10 has a habit of trying to reset PDF association back to Edge whenever it thinks it needs to My Bad as pointed out by @SumatraPeter
The best way to reduce issues for Drag and Drop is to go to sumatrapdf.exe
right click and send to desktop as shortcut on the desktop (NOT tile or taskbar)
then from the desktop shortcut ensure the right-click properties “target” has standard double quote marks and add as shown
"%1" separated by one space.
Now you can test drag and drop on that shortcut
If necessary change its compatibility to “run as administrator”. That setting is annoying as always asking for confirmation, so best if you are a standard user to install without needing elevation at run time. one reason I generally use a portable copy for ease of use.
IF ok then make a second copy of short-cut this time without “%1” for simple start-up or pin that second shortcut to taskbar or pin to start