Update: Yeah, this should be called the draggable-but-not-droppable bug. I can grab something, and the cursor icon changes, but when I hover over a drop location and drop nothing happens.
This happens on all items on the main (building/staff) screen. It does not happen on the (single) formation screen.
Bug still present in Beta 5.1. It took about an hour or so of game play to appear, and then steadily becomes more frequent as play time increases.
Saving and reloading clears the problem.