This happened to me last week on the same day as you posted your question, spooky!

To fix it I found this repair guide, which suggests that the gesture button can get stuck if it gets pressed... wrong.

I used this teardown guide to open the mouse up with an iOpener to soften the sticker adhesive and a metal spudger to peel it up, then broad tweezers to prevent tearing or grease from my fingers, I followed Steps 1-2. Then the first part of Step 3 to remove the black screws. I tested the button and its mechanism. Both seems to actuate fine, so I reattached it without tightening fully to give it a little wiggle room.

Once I reassembled the mouse and used the iOpener to soften the adhesive and smooth it back down the the metal spudger, it worked as good as new, if a little more crinkly on the bottom, which is undetectable in normal operation.