I’m curious what the difference ended up being on your muzzle velocity.
Either way, I’m glad to hear you’re getting better results and that it looks like you can’t blame the powder after all
You’ve probably got around .006” of tension without an expander or mandrel to open the necks back up.
One think extra tension does is make up for uneven contact with the bullet. By increasing the amount, it makes up for uneven thickness of the brass and/or the engendered hardness and thus uneven spring back.
Proper annealing (keyword being proper), and using a mandrel to iron out the inside surface of the necks can really help you get the same result with less tension overall. Just good for thought. I know it sounds like that’s the direction you’re headed.
Good luck with continued success with it!