After dragging, when you release the mouse button, in some cases panel will draw below the other panels in the region.
Whether or not this happens depends on the order of the region's panel list, which importantly is not always the same as the order from top to bottom.

I'm about to solve this, but it's helpful to have a visual record of the problem, so I will record it here:

It's a bit hard to catch in the video, so here's a screenshot of the problem. After you let go, the panel draws below the others.