Isn't "express mode" the same thing anyway? Sounds like what's happened there is the person has just presented the wrong card accidentally even though they did touch in. Usually if you do that you'll be charged the max fare in addition to the correct fare. The inspector may have annuled the incorrect tap after verifying an accurate one though.
The only thing that's different with the DLR is that they can't know you definitely haven't touched in as they can't access all live taps from a DLR station. They can access all live Oyster card taps from the bus ticket machines.
‘express mode’ on iPhone can be set to a different card than the default one on apple pay though.
when a RID is on ‘bus mode’, as the inspector does a download when they get on a bus, they have the list of card tokens tapped in, so when they tap a card it’s either on a list, or it isn’t. when inspected on the ‘rail mode’, the list it checks against is the denied list, as RIDs don’t get a live update of when a bank cards are tapped from the back office, as it isn’t stored on the card unlike an oyster.
therefore, a ‘RID inspection charge’ (RIC) which is either a maximum contactless fare, or currently £16.30 if it is an LU inspector (only LU RCI/Os, no one else yet) is only applied from a RID set to rail mode, as if a card has not been tapped in with bus mode it would be liable for ‘immediate enforcement’ - either a PF or an IR - as the inspector knows definitively if a card has been touched or not. this means there’s no need to undo tap on buses.