In the camera setup above(assuming a static background), the autoISO won't vary so wildly as the matrix 'sees' the background as the primary
exposure reference .. it may vary but a couple of thirds or so(depending on camera settings used).
But, if you do decide to zoom in to a portrait scene, and you have -ve
exposure compensation set, for the portrait shot, you have to adjust the compensation(obviously).
In AutoISO mode that's it .. worry only about the reframed shot now.
But in manual
ISO mode, you also have to worry about what
ISO setting may be needed too(because you're in manual mode).
Aperture Priority mode may not be suitable, depending on
shutter speed requirements and
lenses used .. and camera used too! But it may have been.
7 years back I had the D300, and it's AutoISO is nowhere near as useful as todays
Nikon's autoISO modes, which allow to cater for minimum
shutter speeds depending on focal length and stuff like that!