I've noticed some leakage problems in a few cases. The cause of the leakage was typically not something I could find. Meaning the LIM manifold didn't have a crack, the tensioner housing didn't have any cracks. The only things I was able to determine was the elbow had an odd angle. If when you install, there is any pressure on it (because it only seems to be the top one that will ever leak) they will change shape slightly with heat cycles and that can allow them to leak. I found one or two that became almost 45 degree angles.

A fix that has worked well for me.
Lube both o-rings with oil or vasoline etc.
Install the elbow into the tensioner and wiggle it to ensure some movement.
Bring tensioner into place, engage elbow into LIM and start the bolts on the tensioner to head.
Before the tensioner is tightened, wiggle the elbow to ensure it is square and not bound by pressure of being assembled.

Since having a few problems, I've found that to work each time. Napa selling a lot of them is probably more due to the change in weather and them breaking or leaking vs a defect.