I can't say I've ever experienced that problem on either my 718 or on my previous Porsche's, though I once experienced a similar problem on the drivers door of an Audi I once owned.It happens very often to me.
When i turn off the car and I do the one-touch closing of the windows, they arrive at the top and than go back down as they found an obstacle.
It also often happens when I close the windows using the long touch on the key, and that's more annoying since I've to get back into the car to close them.
Does it happens only to me?
I've tried to set up again full open and full closed position, but the problem (if we can call it a problem) always come back.
If you haven't done so already you could try re-setting the window mechanisms. Page 305 | 2017-2019 718 Boxster Manual | Porsche iManuals It'll be the same procedure for both Boxster & Cayman.
Failing that try a suitable rubber seal lubricant, it's possible that the windows might just be binding a little tightly against the rubber. (Referring back to the Audi it was rubber lubricant that did the trick).