Phase angle between two harmnic orders
Absolute phase is rarely meaningful since is it's highly dependent on your exact definition of your time reference, i.e. what exactly does  $t=0$ mean.
Since your time reference changes from frame to frame, so does the phase.
If you know the fundamental frequency EXACTLY and it's phase locked to your sample clock, you can fix this simply by making the hop size (and the frame size) an integer multiple of the fundamental period.
If it's not phase locked, you can try lining up the start of your frames to the &quot;best&quot; zero crossing in your signal and get the frame length as close to an integer multiple as possible. If your signal is reasonably well behaved and fairly noise free, that should work ok.
If none of these work, ask again. There are more options, but they are also more complicated.