Periodically and any time there is s sudden change in shifting, check for fraying cables. Hopefully you did not undergo too much pain getting the broken cable head out of the shifters, sometimes it is a long and tough job; much easier if you catch it before it breaks off.
The tight curves required for under-tape cable runs makes fraying a much more frequent occurrence so vigilance is key to avoiding unpleasant surprises. We have seen posts on this forum of folks getting far less mileage than you, but of course it is number of shifts not mileage which is the key metric.