Hi John,
I can see now that the pattern of discolouration on its inner surface matches the pattern of iridescence on its outer surface - however only roughly half the vase has been affected this way.
Which now makes me think that either one side of the vase must have been subjected to a high temperature over a prolonged period - perhaps resulting in some sort of chemical reaction or as you suggested things might have gone awry during production.
Perhaps they only got it half right

It doesn’t really bother me - I was just being curious.
In fact I like to see an acquired patina on old things.
I keep telling myself that, but unfortunately not very convincingly

Scott