The wheel is centered by the studs, by the shanks on the lug nuts, and they're not long enough to penetrate the spacers so I think they're "falling down" behind the wheel. Yeah some little sleeves is another approach, that doesn't resolve the weight imbalance issue but that is likely negligible.
I ran each spacer across some emery paper to remove any high spots.. don't know if that did anything. They seem flat, but I haven't measured.
99 uzj100, gx470
05 525EXC, 01 XT225, 07 TTR90, 07 CRF50