I had light off-center strikes after installing my Apex polymer FSS kit, but it turned out to be a combination of worn out recoil spring and crud build-up in the extractor cavity. I had over 20,000 rounds by the time I installed the Apex parts and it was by coincidence that the light strikes started happening just after the Apex installation.

I know it's tough to check, but if you are getting very regular light strikes (once per mag as you said). Go through a magazine or two and just before pulling the trigger, use your thumbs to push the slide forward on each shot. One of the symptoms of the slide not going 100% forward is a light off center strike. The problem is when the striker hits, it drives the slide fully closed so you never know that it wasn't fully forward.

If the extractor hook is full of sooty build up and/or the extractor cavity is gunked up, the case head will have a difficult time riding under the extractor hook and that resistance can prevent the slide from going fully forward.

I assume your striker cavity and spring/guide are clean. That would be another cause of a light strikes.