I've had Logic for many years but haven't used it much. I've had problems with automation and getting the tracks to attack at the same time. Every time there has been an update, I try again to see if the problem still exists. With 10.5, it still does. I've done some experimenting and can't find what's causing the problem.
I put 4 multi instruments though VEP 7. Each instrument had a instance of Kontakt 5 with Berlin Strings. 1st Vln, 2nd Vln, Vla, and Cello, each in their own instance. Each instrument is playing a divisi sustain chord. (Changing articulations doesn't solve the problem.) If I start play right on the beat, one of the instruments will sound before the others. If a solo/unsolo a track, that is the track that will sound first. If I solo/unsolo 2 or more tracks, the first one I soloed will sound first. If I start play 2+ beats before the downbeat, everything is as it should. If I start 1 beat before, the 2nd Vln won't sound at all and the volume on the Vla and Cello is about 7 db softer. Solo/unsolo doesn't make any difference. Starting 1/2 beat before results in results with too many variations to mention. Sometimes one instrument will sound, sometime with lower volume. Sometime with two.
I checked all automation for strays. I have sample accurate automation checked. Unchecking doesn't change the behavior. I've tried various buffer settings. I have no external controller.