Brightness keys show the on-screen slider but brightness doesn't actually change (Lenovo Yoga 710-14IKB)

Hi, the brightness control on my Lenovo Yoga 710-14IKB stopped working recently. Pressing the Fn brightness keys still brings up the on-screen brightness bar and the bar moves, but the actual screen brightness doesn't change at all. It worked fine until a little while ago. Do you have any ideas what to check next?