It's a bug. Have to restart to get it going again. It has been mention here and on motorola forum before. Not many people mentions or notice it but I have had it in all 3 of my Photons. It seems random and happen before and after the update.
I'm not 100% convinced it's a bug. Part of me thinks that it's actually something built into the phone that prevents people from turning on the 4G radio, so that they don't inadvertently cause their phones to continually search for 4G signal (and subsequently drain the battery).
Then again, by what Skrillex had mentioned, he was able to toggle on the 4G radio (even if he's not in a 4G area), prior to the update.
I did notice this early on, when I was in my office, I'd try to toggle on the 4G antenna...it would stay on, and then automatically turn off. The 4G signal is weak inside my building. As soon as I stepped outside, I tried it again and it worked fine.