I have zero experience with this combo ........ but you asked for thoughts:
I think you have a chain of events ..... you correctly identify your brass as new ..... that may be a factor. Also, you are FL each time ..... Can you partial size to the point of having enough neck tension but not leave as much free space around the exterior of the about to be fired casing?
Naturally, you'll need your chamber and brass as dry as possible.
With some cartridges, I'll seat the bullet out to touch the lands in order to keep the base against the bolt face for the purposes of fire forming. You would have to back off powder charges accordingly.
I realize you have formed your brass already but as a stop gap until you get your brass hardened by a few firings or otherwise figure this thing out.
I don't think just trimming your brass over and over will solve this ..... you'll just keep backing away from the problem. Some trimming may have to be done as a part of a comprehensive solution.
Lastly, what about contacting the source for your barrel and ask them for advice?
When I get in a real jam, I'll take just a very few cases and run them out over and over ....... this way you have not messed up so many until you find out what's going on. When you have solved your problem, you'll still have enough left to work with instead of having to try and find more ........ trying times for us gun nuts these days.
Some food for thought .........
Best regards and good luck!
Three 44s
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