Move cursor randomly appears on cell corner hover instead of fill cursor

Hi, I'm on OSX running the latest 365 suite. It's fresh install with no configuration changes, add-ons, etc. 

The behavior I expect is when the mouse cursor is hovered over the corner of a cell the "fill cursor" that is draggable as shown:

Note that the default option "Allow fill handle and drag-and-drop cells" is enabled. 

But inexplicably, some cells will show a move cursor instead when hovering over the corner. 

When you click in that position, an empty hovering box pops up:

If you hit DELETE while the box is selected, it becomes deselected but is not removed from the sheet. 

I uploaded the file to Excel for Web and it did not have this same behavior. So it appears to be an issue specifically with Excel for Mac.

To be clear, this started occurring after some formulas were being worked on. What could cause this? 

Thanks for your help!