While I wait for my first reply to be approved (thus is the plight of the initiate -- and thanks for the hard work, mods), I did some testing on Stelgaer's Pride.
"Common" Breastplate equipped
got near unique chest but not close enough to trigger interaction prompt
went back to camp and upgraded to "Fine" breastplate
went back to chest and opened to find "Fine" Stelgaer's Pride
(reverted to save) "Common" Breastplate equipped
got near unique chest, brushed up against it, but did not trigger interaction prompt
went back to camp and upgraded to "Fine" breastplate
went back to chest and opened to find "Fine" Stelgaer's Pride
(reverted to save) "Common" Breastplate equipped
got near unique chest, triggered interaction prompt but did not open the chest
went back to camp and upgraded to "Fine" breastplate
went back to chest and opened to find "Common" Stelgaer's Pride
I believe this is hard evidence that the chest contents are rolled when the prompt is displayed, whether or not the prompt is a preview of what's inside.