I suspect neck tension and actual case size. Hornady and Stoney Point before them, that their modified cases should never be resized. The case body and shoulder of a modified case is sized to SAAMI spec and the neck, a smidgeon greater, so to provide a friction fit that will allow for removal from the chamber and an accurate measurement gained. Your fired case, despite filling your chamber, is “too slack and out of spec” for the job.

The cost of a “factory” modified case is not just the drilling and tapping...