I got the same problem recently.
CS-R701 cassette, maybe 3000km on a first wheelset, then upgraded to a new wheelset for 1000km until it developped the same play as you. i noticed just before leaving for the alps and the play was gone for 2 days before returning. a new SLX cassette on that hub had no play, the cassette also had play on 3 other wheelsets i tried. replacing the lockring did not help. i bought an identical r701 cassette and play is just gone now, so the cassette is definitely at fault. there is no obvious damage to the cassette or dent to the back of the spider. still a mystery to me
do you have enough compatible parts like me to troubleshoot and confirm the cassette is at fault?