You can't rotate a text with the Rotate tool when Snap to grid is on.

Please watch the GIF attached.

Then try to replicate.

Open the attached document.
Enable Snap to Grid.
Select text.
Grab Rotate tool.
Hold Shift (or even do not).
Try to rotate the text.
Failure?
If yes, move it a bit to the side (with snapping still on) and try again.
Experiment a bit with different positions.