Are precision rimfire rifles ammo picky?
Yeah. They require the uniformly best made cartridges available.
No rifle can fix problems caused by ammunition
that has variations in assembly, components, primer/powder amounts,
chemistry or has been subjected to rough handling, extreme temperatures.
Asymmetric bullets and variations in center of balance
will affect how the bullet reacts with the rifling and atmosphere
after it exits the barrel. If a box of cartridges produced good results,
it would do so from any properly built rifle.
If it produces strays and vertical spread,
that'll happen also with other competition grade rifles.
No two cartridges that roll off the assembly line are identical.
No two boxes, bricks, cases of cartridges are exactly the same.
Variations in trajectories are caused by lack of uniformity, cartridge to cartridge.
Get a run of well made cartridges, similar in quality,
you obtain similar behavior and trajectories.
If you are sold cartridges that have issues with components/assembly,
it doesn't matter how good the rifle, the ammunition will cause problems.
Well made cartridges produce predictable trajectories
when used in any competition grade rifles.
Cartridge quality as it made, is not a constant.
Some boxes, bricks, cases will be better/more uniform than others.
Get a bad box, the rifle gets blamed for having a brand preference.
Get a good box, then the rifle "liked" that brand.
Failure to recognize the existence of variations in cartridge quality
and thinking that every cartridge is identical,
is responsible for the concept of rifles being "picky".
Uniformly well made cartridges will produce better results from all rifles
than cartridges with issues behave in those same rifles.
My rifle likes Lapua and hates Eley but loves RWS
can change with the next purchase of ammunition.
Now it likes Eley, loves Lapua and the RWS reeks.
Cartridge quality is not the same, moment to moment during manufacture.
Batch testing at the factory does not catch all the problems.
Welcome to the assembly line lottery.