MIG wrote:

I was told one had to always specifically touch in and out for the DLR,
hence the tunnel readers at Bank.

The explanation for those does seem to be the possibility of arriving
via the lift without passing a gate. I've never had a problem with
not touching there. A touch at the DLR one end and LU the other end,
with no touch while changing, has always resulted in me being charged
for one LU/DLR journey at the right price.

Last week I saw notices by those readers which suggested that they're for
Oyster users going out of zone, not people who've come down in the lift.