Starter Problem

I’ve had my 28’ Commander for about five years now and have never had a problem with a starter motor. This past winter I had both Q engines rebuilt which included rebuilding the starters. After getting the boat back together and starting the engines only a couple times, the Bendix on one engine broke. The starter spun freely but the drive gear would not engage the flywheel. I took the starter out and had it repaired at a local shop. Re-installed it and it immediately broke again. The starter is back out and I’ll have it fixed but I don’t want to put it back in until I figure out what’s going wrong. It’s the original Prestolite starter and the original flywheel so I don’t think it’s an incompatible parts issue. The only thing that’s different is that I noticed the engine builder did not put back on the brackets that support the rear of the starter. I scrounged up a couple and I’ll put them back on both engines but it seems unlikely (at least to me) that the missing brackets would cause enough flex to damage the Bendix. I hope I’m wrong. That’s an easy fix. Other than that I’m out of ideas. I’ve been told that those engines did not use shims under the starter and I had two mechanics say they saw no unusual wear pattern on the gear. Any thoughts on what to check next? Thanks.