does it change through the travel on a HT
If you take the same frame and put a longer fork on it, then the reach will decrease, as the frame lifts at the front end, you tilt the saddle down to compensate – both of these result in the effective top tube, and reach reducing.
However, it’s not true to say that the more travel a bike has the shorter the reach. Indeed, this is actually the opposite due to trends Enduro / AM bikes have longer reaches than XC bikes – in part due to XCers wanting longer stems and narrower bars, and Enduro’ists wanting shorter stems and wider bars