Wow. I think you have found a bug in the firmware! I have tested the described scenarios using D7k in AF-A mode, and yes, it behaves nearly like you describe it, except one tiny thing: if you focus on some close object, hold the button half-pressed, then move the focus point to the **nearer** object, then you can go to the infinite objects, then back to near ones, then far ones etc - no problem, all works like expected. It seems the tracking logic code has a bug - to start tracking, it needs first to "see" a change to a shorter distance, then it tracks OK in the whole range.
'For an idea that does not at first seem insane, there is no hope.' -- Albert Einstein