Well if it's the same atty that 18 tastes good with and 24 less so, then it can't be the atty, although if we're talking two different atties then that may be different. Use the same atty if you want to find out this difference.
It is true that nic does affect the flavor and will add a peppery taste to it, and there's enough of a difference between 24 and 18 to notice it. Zero nic will give you the best flavor of course, it's a matter of getting the nic you need though which is why we
vape in the first place, if 18 is enough then great, if you need more than you need more and whatever flavor you get is what you get. You could always try different juices though if you find you don't like the taste of 24,
tobacco juices in particular can be harsher, where something candy flavored can mute the taste of the nic better.