AFAIK all 12 spd cassettes require an 11 spd hub. Only 11spd mtb cassettes (defined as any cassette with more than 34 tooth large gear) requires a 1.85mm spacer to fit an 11 spd hub but no spacer on 8,9,10 spd hubs. But all 12spd cassettes require 11 spd hubs and hence the inclusion of a spacer is strange. Then again it could have been a very thin spacer to account for bad manufacturing tolerance so without seeing the bike it's impossible to know if this was a genius fix or a WTF.
Of course this assumes we're talking Shimano HG freehub and not Sram XD or Shimano's new Microdrive.