I-Home improvement

[Countmein]

I suspect that the seam doesn't fall on a joist to begin with
I think you are talking about the seam on the backer board.
I took it that he was talking about the seam of the subfloor material, ( the plywood ).
But it will be a pain screwing down the floor into the new bracing.
You don't have to screw the floor to the new bracing. Just the bracing to the floor joists. That alone will strengthen that entire floor area.
I wish I could pull up a picture what I'm talking about.
If his plywood is moving up and down on the floor joist, that's another matter. I suspect that one floor joist where the plywood seam is, is moving up and down ever so slightly.
The easiest fix would be just replace the broken tiles and hope it doesn't crack again until he has the house sold! lol

Tile is great on concrete, but on wood floors there can't be any movement what so ever.

Count