CS-M760-9 loose on fh-m775 ?

As from the title, XT cassette M760 is loose on the M775 freewheel. It is lacking some 3-4 mm (without precise measurement) so I can't tighten it with the lockring. All sprockets are in place.

Is this behaviour anticipated? I am thinking a regular spocket spacer from an old cassette would fix it (plan to put it on the inside side), but just wanted to check if anybody had some experience on this.

Thanks