Shortly after I sent my previous post, I sent a similar inquiry to Missouri Bullet Company. The last thing I expected was a reply after hours on Good Friday, but Brad responded almost immediately and was gracious, helpful and concerned. It's the best service I've ever received from any company I've ever dealt with, and they've got my vote.
I bought a bullet-puller today so that I could research this further. After pulling a couple of bullets from the rounds in question, the bullets measured a perfect .358, and I still haven't found the actual root of the issue but it's certainly not the bullets.