It's worth noting that the plastic bolt is not the primary method of retention here. It is the pinch bolts which clamp the crank down onto the spline that does all the work. If they're tightened correctly they should not come loose and the plastic(or alloy) bolt should not even come into play.
I recently swapped cranks, i have tensioned the pinch bolts to 13nm and checked/re-tensioned after a short ride and then again after each ride until i was no longer getting any any movement from the bolt before the tension wrench clicked off.
Various factors will cause the pinch bolts to lose some tension after the cranks are installed, the components will settle in after some riding and the bolts may "stretch" a little as well. I think it is important to check the bolts and tighten a several times to ensure they stay nice and snug.
I recently swapped cranks, i have tensioned the pinch bolts to 13nm and checked/re-tensioned after a short ride and then again after each ride until i was no longer getting any any movement from the bolt before the tension wrench clicked off.
Various factors will cause the pinch bolts to lose some tension after the cranks are installed, the components will settle in after some riding and the bolts may "stretch" a little as well. I think it is important to check the bolts and tighten a several times to ensure they stay nice and snug.