To reproduce:  go here and click on "Settings". See the slider at the top.

Click the little up/down arrow buttons. The number to the left of the arrows changes.

Expected result: the slider knob on the right should also move, in sync with the numbers.

Actual result: the slider knob does not move.

Consequence: the slider knob is out of sync with the numbers.

Note that the slider knob does move if you use the up/down arrow keys. It is the arrow buttons that fail to update the knob position.