I had similar issues with my initial Rimx barreled action in 2020. The cases that fired had what looked like cushioning and I asked aIbout hard primer strikes since they would fire in another rifle. Coordinated with Ray and examining the chamfer, we suspected it was too large/deep. My local gunsmith coordinated with Ray to cut the chamfer to proper Zermatt specs, problem solved. 5 more barrels later and never saw the problem again. There is another similar case back in the 2020 discussions. Same timeframe as yours.
I'm sure John would have taken care of it but, after getting lost in the mail (Covid timeframe) for 2 months, I decided to get it done locally.
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