They blamed the marks on the chrome on tarnish from salt on the road (in April!) but it looks like this is a Triumph “thing” as other models with similar finishes are also suffering from the same. ACF 50 treatment seems the only cure / prevention. At the time of the dealer closing for lockdown they had not received an answer from Triumph on the exhaust markings but they cleaned them off at the first service.
I will be taking it up with them once they re-open.
Good luck! Keep us posted please.