Axes.indicate_inset_zoom just creates a static overlay which does not adjust when inset axes ranges are changed. Additional calls to Axes.indicate_inset_zoom will create new overlays. It is expected that the result of this

ax.indicate_inset_zoom(ax_inset)
ax_inset.set_xlim(new_lim)

and this

ax_inset.set_xlim(new_lim)
ax.indicate_inset_zoom(ax_inset)

are exactly the same.