I don't usually promote guys if they aren't hitting well. Let the player tell you when he's ready with his stats. As others speculate it could have been a talent drop in AA but hopefully your scout would have told you that when re-rating him. I straight up ignore ST stats, 30 AB doesn't show anything and lots will have been against AAAA players or kids.
Also, not to open up the whole SABR can of worms, but a low average guy can still be providing value with homers, walks, and so on that don't show up in his batting average. I assume if he had 50 homers you wouldn't be complaining about AVG
