So something I keyed on in your post was "was serviced with no issues". Did you service it or did someone else? Have you checked the oil?
Shift with authority. There is not much else you are going to be able to check if there is indeed something wrong with your transmission in this bike.
I took it in as new tires were needed. She got a clean bill of health after her 36k servicing, and no issues were found. Hopefully she holds together for a few more weeks.
My 13 did it a few times while I was getting used to it.
Now I shift it with a full stroke. Its a habit, and so far no more problems.
The bike had 8500 miles when it was doing it. It now has 27k.