Flooring Tile - Interface failure: wood trim/ceramic floor tile

Not long after our contractor completed the remodel of our kitchen and bath, we noticed that the grout lines at the point where the ceramic floor tiles meet the older hardwood in the upstairs of our house have "failed" and have basically fallen out. It's happening where contractor nailed a piece of floor matching oak trim nearly right up against the edge of the tile and left a quarter inch gap for grout.

No where else in the house has this happened, including the new hard wood/ceramic tile floor downstairs. We've tried replacing it with the identical sanded grout used before, following the mixing instructions exactly. When that failed again, we tried the unsanded version. Still failed. We're running out of ideas.

We suspect it might be due to the older floor flexing. Is there anything different we should try? Caulk perhaps?

We're frustrated and would appreciate advice.

remove the grout at the transition and use color matched sanded caulk.