So it transpires that the non-Zone 1 fare is still charged when using the ACY-UHL OSI. That surprises me but perhaps it’s due to limitations in the way that the Oyster card stores/processes the tap data?
I’ve read of similar things happening (albeit overcharging in that case) when doing an OSI between Blackfriars NR and LU and this attracting a mixed mode fare even though this is only advertised as applying if changing at other London Terminals.
I wonder if the ‘bank errors in your favour’ as above would happen if using contactless. I’m guessing not, since the back office system is more sophisticated?