Official comment
Yeah, I aways have my lock enabled to prevent any accidental load, but that's still a problem if it completely unloads the deck.
It seems it doesn't matter if you use the keyboard shortcut, mouse drag, or controller load to pull a track to the deck - it works as long as you've loaded the track from the collection or a playlist. If you load it from the Related Tracks, it doesn't work at all. If you then load another track from the collection after loading from Related, then "Undo," it empties the deck.
I'll pass this along as clearly there's inconsistencies as to the functionality here.