That's pretty much in line with what I've noticed. Dianthus is actually the one I espsecially noticed with the D-class. I actually do hit it with that amp planetside often enough, but I've noticed more of an all or none effect. Sometimes I won't hit a single claim of it, and other times it's back to what I consider normal for that area. I remember one time having almost an entire amp of nothing but Dianthus and Devil's Tail on a D-class.
It could be that higher level amps do alter the average find rates for specific resources that results in an lower average MU. I'm seeing a trend like that right now, but these resources also tend to be variable over time anyways, so it could be we're just seeing that effect and need to burn a lot more amps over time (e.g. many days or weeks). When we're using bigger amps that don't last as long, it's also a lot easier to see trends that might just be part of randomness or a current swing in loot.
I might start another set of runs to run statistics on like my finder decay affecting claim size experiment (still no difference there after 1000 claims). I'm eyeing something like three identical finders having a level 2, 5, or D-class and rotating between each of those after each find. Calculate average MU for each amp's claims and see if there are any statistical differences.