I just tried some of mine on my bathroom scale, I now have an imprint of a 9mm in the heal of my hand. OUCH!. (I couldn't resist)
I got 55lbs with several head stamps before it hickover sideways and no set back. I think a cheap arbor press would be in order for testing this.
I don't know what the pounds considered safe would be either but with fingers, yea, definately not enough neck tension.
My thin cases resize at .369 and neck size at .372, but mine have been shot so many times that they are work hardened to the point they don't stretch as easy as soft brass does. My bullets also mic between .355-356, mostly .356.
Your sizing die may be a little over sized also that would account for why it isn't catching the thin stuff or mine is undersized and works better with the crap brass their selling today, mine mics at .382, or your running your flaring die to deep and undoing what the resizing die just did. I don't know what a resizing die is supposed to mic at inside, maybe someone else could tell you that.
I'd like to know what the 9mm resizing die is supposed to mic at inside, if mine is undersized, I need to stop replying to posts like this as my situation won't be the same as most everyone elses.