Belt too long will break them far faster. The tensioner arm repeatedly hitting the off stop will break arm clean off. Make sure when belt is changed the tensioner is running in the range marking cast on tensioner, if not then wrong belt.
Ford branded tensioners actually suck, they don't last as long as others. I've had new ones fail in less than 6 months, there are reasons why we won't go into here. Gates are good.
I've been running my 2 Foci for years with no water shields, the tensioners that I've rebuilt myself seem to last forever like that.