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Lockring tight, no space for a spacer but the 3 largest cogs which are on a carrier work loose, help
You’ve probably got a notched freehub which prevents the whole cluster being preloaded, or the 11T is on wrong which would also prevent the whole cluster being preloaded.
Other possibility is that it’s a road 11 spd freehub like a Hope, and needs a spacer behind the cassette.
Yeah, I felt some play in my cassette last week. Tightening it did nothing. upon removal of the cassette, the freehub is notched big time.
Sometimes light touch with a file might give it a bit more life.
If it is a hope (or any other) alloy freehub, check it for cracks - mine did this - the crack was tiny but caused what is happening to you