I mounted a Maxis Grafter 2.10 on an Urban Echo BMX rim. I built the wheel. I know it is true. Minimal hop (a little due to the seam) and true laterally.
When I look at the wheel with the tire mounted I see noticeable hops which seem to occur because the tire is not evenly mounted. I can see a seam on the sidewall that pops in and out of the rim indicating it’s unevently seated. I tried everything, including soap on the sidewalls. That’s not the issue as it’s a relatively loose fit. I tried inflating a bit and working my way around getting it even, but the entire tire just doesn’t want to seat properly. I don’t know if the tire is bad, or it’s a bad fit between the rim and the tire. I think it’s probably very close to this rim, though I no longer see the rim I ordered over a year ago:
Dunno if it’s what you’re describing but I’ve noticed especially that my 20" tires develop a bit of weave because of where the the tread is in relation to the cranks. When we push on the right pedal, the unicycle wants to turn right and the tire resists, which puts a twist in it one way. Half a turn later, a different part of the tire resists turning the other way and twists in the other direction. Before long it takes a permanent set.
It seems to happen more with fat tires that I’ve ridden at lower pressures, and maybe that I’ve ridden at low speeds with big pedal inputs and quick changes of direction. I really saw it on the Onza Sticky Fingers tire that came on my 20" Impact. Rotating it on the rim ought to help but it never worried me enough to bother.