The same thing happens to my HTC one M8. It started about a month ago. When I finish a text, the first word of what I've written always shows up in the dialogue box. I am using the native text application that came with the phone. I am also seeking a resolution to this problem. I have done a Factory reset and an Android system reset with HTC. I am running 4.4.4. Any suggestions?
Sorry you're still having this problem. If Sprint can't help you fix it, I would contact HTC, although it sounds more like a network problem to me. Maybe try a different messaging app, to see if it's app specific or not.