At first I didn't notice, but then after farming lvls for a day to get the apparel event keys I kind of got the impression that didn't saw a bonus item from caches, tought to myself ah maybe it dropped but I don't remember or something.
But got the bug behind my ear and started to note down just to make sure, and here are the results after a few days:
21 caches 0 bonus items
Well, that's your "50% chance that a field proficiency cache drops a bonus item"
Either is bugged (Most likely) or I have been on extreme wrong side of luck, conidering that most of the time before the update I got a bonus item I am more for the bug version.