I had a similar problem with 105 shifters. About 20 minutes of upshifting, downshifting and grabbing at the end with tweezers and holding onto it while shifting eventually sorted it out.
My best guess as to what is causing it is if the cable isn't moving freely (e.g. the cable outer near the derailleur is rusted or something like that) then when you upshift, the cable bunches up in the shifter and gets damaged over time.
You can get
exploded diagrams on the Shimano website if you need to take it apart to get the cable out.