Ours was ply on the joists, tiled, and the join was on a joist, but the perpendicular join was crossing the joists and that's the area all the grouting cracked.
So I'd Agee it'important to have a stable floor before tiling.
Admittedly our joists are a little bouncy due to being chopped half way through for a 40mm waste
If its not going to cost too much or be a major disruption, then for peace of mind maybe get the T&G done. That is unless your confident there is no bounce/flex/movement in the boards that will cause issues down the line. One can only assume the fitter has had issues before that then reflects badly on his work or caused a further visit to repair (possibly at customers cost).
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