I do not pretend to know what is going on.It seems you have found some lighter bullets that work.
That's good.
One factor with lighter bullets,especially the sleeker ones with boat tails,the actual cylindrical bearing surface of the bullet gets pretty short...which translates to "tippy"
One of the places that CAN show up is in how straight your bullets seat in the case neck.
Straight,concentric ammo shoots better than ammo that is a little wobbly.
Some little detail of how you set your die in the press,or the quality of the neck chamfer can make a difference,as can a floating bushing seater die.
If this is a new rifle,consider this:After the barrel is rifled and the bore is finished,the chamber is cut by a reamer.
The reamer is rotating across the rifling lands.Every cutter,no matter how sharp,leaves a burr on the downstream edge of the cut.
Some shooting will wear those off eventually.
IMO,those burrs on the rifling in the leade ,leftover from chambering,are about 90 % of the controversy over "break in".
In full agreement with Mr McMillan,there is nothing the shooter will do to improve the quality of a good barrel.
However,with respect for Mr Krieger,it is not so good to leave the copper flakes,dust,and chips these burrs carve off the bullet laying in the bore for the next bullet to run over.
It may well be you have a little more burr trailing off one land that is grabbing,tipping,bullets with a very short cylindrical bearing surface.
You may well find,as things wear in a bit,that your rifle becomes less picky.
But,I have been wrong before!