SLX 12 speed drivetrain issues

Long story short...I've just swapped out my worn 12 speed SLX cassette and SLX chain with like-for-like new.
It was shifting ok but as I have some big trips coming up thought it was time to refresh it.
The chain is now jumping around on the sprockets where the spacers are.
They spacers are different thicknesses - there's even a YouTube vid highlighting the issues caused by having the spacers in the wrong positions, causing jumping chains and people pulling their hair out (not literally!). Mine are correct as I've triple-checked. The mech or hanger haven't been damaged since the last ride. Someone has suggested jockey wheels or chainring, but wouldn't they cause issues in the other gears too? Not sure what to do...I have another unused hanger (Privateer 141), new XT chainset and new Deore mech, but hope there's something dumb I've overlooked before replacing that lot...