If the cassette is properly installed then it’s a freehub issue.
Edit- try moving the largest and second largest cogs away from each other with your fingers. If they move easily, something is wrong with the installation. If they’re tight, it’s the freehub.
My 11sp Shimano cassette on my 11sp Novatech hub is loose. It can't be tightened any more than it already is. There is significant movement.
There shouldn't be a spacer behind the cassette when it's 11sp, right? and the cogs are spaced correctly with no spacers missing in-between.
Does anyone know what might be going on here?