It does make sense, the manufacturer was very clear on their reasoning. The 18mm marine ply on a bed of adhesive I've put down seems like concrete, can't see that flexing!
Thanks for the quick reply. Yes, probably should have started a new thread. I spoke with the supplier, the reason they want you to use a polymer adhesive is actually to allow it to flex, its kind of opposite to what I thought. As the ply is screwed to the floor its rigid, if the resin stone tray...
Recyling an old thread, but similar question... Old house with wonky floors, now within 10mm level on a 1200x800 base! Plan to put 18mm marine ply on top of 22mm floorboards, screwed down then bed my resin stone tray onto mortar to get perfectly level. I also wondered about rapid set tile...