Firstly, apologies for taking so long to come back on this. Life gets in the way sometimes.

Anyway, the problem is solved. I'm embarrassed to tell you what was causing it in the end.

It was an old, worn-out mouse. Simple as that. Holding the button down to drag things was failing & it kept dropping stuff. I figured it out when I came to borrow that mouse to help a friend fix a problem with her laptop. I swapped it out with a new one & it now works fine.

Try the Esc key when that happens. See if then it works as normal.

Yes, in Now Playing. I've restarted the computer several times since I first noticed this.
It's weird. I click & hold down on a track, or selection of tracks. Every so often it works as normal, but as often as not it just highlights more tracks as I drag, or deletes everything above the one I've selected & starts playing that one. Maybe I need to replace the mouse battery. I'll try that.

Nope, wasn't that.

Are you working in the Now Playing? Have you tried a PC restart?

I don't think there is a setting to limit Drag & Drop in the Now Playing.

I have AutoDJ set up to keep a constantly updated playlist of 10 random tracks. I often then load complete albums I fancy hearing between every couple of random tracks, by loading them as "play last", then dragging them to wherever in the playlist I want them.
Recently, dragging & dropping within a playlist has simply stopped working and I can't figure out why. I'm pretty certain it must be my end, as I'm sure I've done it successfully since updating to 4.1.24.1833, but if I changed a setting somewhere.which has brought this about I am at a loss to find it now.

My 32 bit Windows 10 installed updates very recently, could it possibly be that?