I'd like to better appreciate this problem – I don't understand it
are you asking about the 'update' issue? If so, i can just explain it – my real concern is really that I don't want to accidentally create stored fields!
with the update feature, it reads out all stored fields and then writes them back to the index... so if you update the bbox, that would write the various subfields as if the original request had them. I'm not sure the exact behavior, but it seem unpredictable and will depend on field ordering etc. If the field is multi-valued it may work, but that is also an incorrect assumption for the normal case