This is the case for getting mouse coordinates as screen coordinates, as coordinates in world space, and as mouse axis values (tested in blueprint and in C++).
I have created a repro project, where I execute some silly code to drop the framerate a little (it seems to occur more under these circumstances, but sometimes even at 60 fps) in the game mode code class. In the player controller blueprint, I print a string if the current mouse coordinates are equal to those received in the previous frame.
To see the difference, move the cursor around without any buttons pressed, and observe how the string isn’t at all printed. Hold down any mouse button and watch it appear frequently - the mouse coordinates aren’t being updated properly when a button is held down. You can print the coordinates directly and watch them not change, as well (same with the axis values - they’re often ‘0’ when you’re actually moving the cursor).
I can’t test this on a newer version here at work, my apologies.
Let me know if you need anything else.
EDIT: DxDiag attached.