Connecting to my PC, running at 60fps (at any resolution) and enabling Auto Motion Plus evidently lowers the framerate to 30fps, unless I'm moving windows around or scrolling through a page.
Here's video evidence (shot at 60fps, shown first in real time and then slowed down)
The first row is at 60fps, second at 30fps and third at 15fps Here's the site for anyone who want to try this out. https://www.testufo.com/
This happens both with and without Game Mode, and in every Picture mode. It's like the TV thinks the signal from the PC is at 30fps until you move some interface elements at which point it realises it's 60fps (pressing the centre button in the remote shows that the input is 60p even when it's effectively showing 30)