to be honest i have only plyed small bathroom floors before and ive used 6mm plywood before ive not had any trouble with it
same here small areas, but tons of screws.
only because of height . with 12 mm
but have made a fuss of it .no movement before tiling.
never had any probs.
(when you think they addvertise addhesive to go stright on to tounge and grove floor boards) with no ply at all ...