Delta explained to me that there is no place in the actual reservation system for a middle name because the reservation system is pre the days that passengers used middle names for travel (this seemed odd because I thought they just went to a new reservation system). This leads to middle names ending up all over the place depending on who enters the reservation - travel agent, delta agent, delta system that pulls from internet reservations etc. Your reservation somehow pulls into a passenger record which is what you see and what is sent to tsa for pretravel. In theory if your profile is correct IT somehow makes sure your passenger record matches your profile and gets your middle name in the correct place. This does not always happen but you can go in to each reservation and check and correct if it is not where it needs to be. What your boarding pass reads (ie where and how you middle name appears) makes no difference for TSA pre it is all about what is entered in your passenger record

Not sure this is true but this is how it was explained to me an old reservation system with a new interface which don't always work together. Most important thing I got was go i on each reservation and check the passenger profile at least 72 hrs prior to your flight especially if you make reservations multiple ways