Does the selector have a little slack when the detent is engaged on SAFE? I assume you mean when the weapon is on SAFE, you can still pull the trigger back a little, though it won't fire? JP's selector is adjustable, whereas ours are not, the slack is then wholly dependent on the trigger's rear extension's geometry.
The slack is normal. Depending on the height of the trigger's rear tang, if it's low enough, the selector's center flat will bar it from pivoting up (no slack), if it's taller, then there will be a little slack before the selector's flat comes in contact with it and stops its movement.
We've spent more time troubleshooting triggers that are in the form of trigger packs than ALL factory and other conventional triggers combined. The pattern here is the trigger's rear extension / tang that looks and behaves somewhat differently than triggers based on original trigger designs.