I think the glove thing is 100% related to the new foreign-substance crackdown. The ump thought his first glove looked too shiny. Somebody, maybe Beck or Mize, misinterpreted that as too light, but obviously that isn't right since he switched to a lighter glove. The ump showed good discretion, imo, by not accusing him of anything or even checking if the glove was sticky, he just told him to change it.
But, I think there is a chance that Mize's normal glove has a shiny/sticky coating that helps him get a better grip and he is going to have to learn to pitch without it. He did yank a few curveballs last night - one led to a run - possibly indicating grip issues but be seemed to adjust as the game went on.