oh my yes it is in the MLB Rule Book Flint.. the ball in the receiving fielders glove HAS to be secure at the time of the called safe or called out determination by the arbitrator either at the sacks or the dish. it has been that way since, well, before I started to follow the game for sure....and that was a bit ago. if those long time rules have been modified... I am not up to speed then I guess.
I did just post the rules regarding a catch, and I’m asking where anything is mentioned about a ball moving while being in the glove. It’s impractical to expect a ball to hit the mitt and just “stick.” It doesn’t happen. Bobbles are generally obvious. This was far from it. Force plays are the only relevant situation where the definition of ball security would come into play, and I’d like to know the answer to this myself. I suspect it’s not something anybody can definitively answer. I’d be willing to guess there’s some ball movement on a majority of force plays, and the case could be made that if the ball never leaves the glove (it didn’t) then the catch is made when the ball hits the glove. I mean, we’re talking in hundredths of a second here.