The giveaway is the case coming free after waiting a few minutes. That to me is soft case expanding with hot load, then when it cools down it releases enough to extract. Ive had it before.
You can try different brass. I used to get that with a .270 when I used Fiochi brass and the time honoured 60 grains of H4831 with 130g SP. Changed brass brands and all fine. Same velocity and accuracy.
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