Assigning a keyboard key to an MX Anywhere S3 side button results in a single activation of that key when the button is pressed --- but does not behave like the key is held down when the mouse button is held down.

This behavior is inconsistent between devices -- the result of operating the control should be the same whether it is being activated via the keyboard or the mouse.

As a consequence this mouse is as frustrating as the MX Keys backlight keyboard which doesn't have a duration setting for the backlight. The hardware seems fine, but the software is lacking in silly ways which limits the device's usability.

The G Hub software works as expected when assigning keyboard keys to mouse buttons, you should import that behavior into the Logi Options+ code.