Hairline cracks in new tiles

Wood is a live material and will expand , shrink and twist. You say the floor originally wooden. If this room has been a bathroom for much time. previous history of leaks etc could damage the floor joists. I would have suggested lifting old floor boards, checking condition and security of joists. replace old boards with interlocking floor boarding screwed down firm. when tiling on wood substrate use correct adhesive which has a degree of flexibility. Same applies to grouting.

It is likely to be happening due to movement on the substrate. Laying tiles to wooden substrates can lead to a problem once the wood expands and shrink with changes to temperature. Installation of uncoupling membrane should reduce or even eliminate the movement accordingly. I am afraid the problem may persist or even increase within time.

This problem will not go away,will only get worse it has not been installed correct,it will all need removing and either a uncoupling mat or better still overboarded with 6mm cement board,the bathroom fitter is wrong.