To David,
I have also had this experience too you know but in a different location. I was travelling to Leicester Square (zone 1) from North Greenwich (zone 2). I went to Canary Wharf, did an OSI to the EL station and went to Tottenham Court Road and then took the Northern line. As Canary Wharf is in Zone 2, it had charged me the peak fare there when tapping out. Just like your situation, when I tapped out at Leicester Square it had STILL charged me the peak fare. I don’t know why this is happening tbh.
I have also made a post on this which is titled ‘Zone 1 peak fare charged in the evening peak’ where Mike has already replied to my post as this was made earlier. If you want to go and check that out you can do that too. Or wait for Mike to reply here anyway.