The UCLA Bruins held their Pro Day on Tuesday, and there were over 90 scouts in attendance. Unfortunately, they did not get to see linebacker Myles Jack run the 40-yard dash or shuttle runs. Jack did the vertical (40") and broad jump (10'4"), as well as positional drills, according to Rob Rang, but he is saving his 40 for a personal Pro Day he will hold on April 1.
Jack suffered a torn meniscus this past fall, and was recovering from surgery at the Combine. He was fully cleared last week by his UCLA surgeon, but my guess is he did not feel like he had sufficient preparation time to run the 40 at the school's Pro Day. I suspect there will be plenty of interested parties on hand for Jack's personal Pro Day on April 1.
Jack is viewed by many as an inside linebacker, but he said at the Combine that NFL teams talked to him about a variety of positions, including various linebacker roles and safety. User Reasonable Opinion recently put together a fantastic FanPost about Jack, body type and defensive evolution. I highly recommend you give it a read.