My solo air sektors do the same thing, sort of 1-2 mm vertical play when they’re at full height. Stripped them down and seems to be slight movement with the solo air shaft, think it’s something to do with air passing to the negative side. There’s no play in the bushings and they work fine when compressed a bit, but it’s annoying on steep climbs when the forks unweighted and you feel the knocking. No idea how to sort it though