Check your TSA Secure Flight Data
Something similar recently happened to me. I've been TSA PRE and had a redress # for may years. Last time through PDX I got the dreaded SSSS and the lovely patdowns that come with it for the first time ever. Turns out someone, probably the travel agent, entered my first and middle names in the first name field and left the middle name field blank in the TSA secure flight data. Note that all the same info on the ticket was correct.
The lesson here is to always login and confirm the secure flight data is correct. I'll be doing that 100% from now on.
nrgiii