Revit MEP - WTF Department...
He had a Revit Architecture model linked into his Revit MEP model, and any time he placed an air device or light fixture in his model, after moving it to the ceiling elevation, in the ceiling view he couldn't select it except with a window selection and he couldn't align it to the ceiling grid unless the flow arrows were turned on.
I verified the same behavior on a test file of my own. WTF??? As long as you're not in a view that shows the ceiling grid, you can do whatever you want with the device - but you need the grid visible to align it!!!
Here's the workaround - simple really - in the ceiling plan, simply set the Model Graphics Style to Wireframe and all will work normally.
But one has to wonder - WHY?