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I specifically wanted the mastic, instead of the thinset, because it's far more forgiving on a wood floor that's not stiff enough. The thinset provides no capability of movement............the mastic effectively provides unlimited capability. The floor is a single sheet of 3/4" plywood with a sheet of 1/4" luan on top..........not very stiff.
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Some dry set thinset now comes with flex-type capabilities. --The higher grades.
Your substrate's deflection is not supposed to exceed 1/360th degree of span. In addition, tiling to plywood, wonderboard, durock, hardibacker board, whatever, without a vapor-moisture membrane..........that's a no no too. --None of those products are waterproof.. Any plywood should be double thicknesses -- 3/4" marine-grade. Mastic is usually a disaster, and a waste of money..