Calendar pops up and then immediately disappears

My bookings calendar will pop up when a customer selects the date picker boxes, the calendar pops up for a split second, then disappears.  I have cleared cache and tried other things, but the problem persists.  Here is a particular product you can try it on.  https://www.specialtyautotoolrental.com/tools/audi-4-2-liter-cam-and-crank-timing-set-tool-t40070/

Oddly enough, sometimes it will stick and stay up, but most of the time it disappears like a thief in the night.  Love the plugin, been using it for years!