I had an old steel bike that gave me similar grief once. After puzzling for awhile I found that the slot in the seat tube had closed up completely even though the clamp had enough gap left.

I imagine it was from the softer steel getting thinner and expanding the circumference of the tube there. The seat post clamp had been seriously over tightened very many times. (back in the days when I believed everything should be tightened to just before the point of breakage.) Taking a file to the slot in the seat tube and making it a little wider solved the issue

Don't know if that could happen with titanium. Never worked with it myself.