In case anyone was wondering, this is how credit is deducted from your Oyster card on the Emirates Airline (attached).
So it would appear the entry barriers deduct £3.20 immediately and the exit barrier deduct £0.00. This was also confirmed by the balance shown on the gate: £0.00 on the entry touch.
This leads me to assume that's why you can't do a return on Oyster without touching out. Because you'll only be charged £3.20.
These deductions also don't show up on the online journey history.
So it would appear the entry barriers deduct £3.20 immediately and the exit barrier deduct £0.00. This was also confirmed by the balance shown on the gate: £0.00 on the entry touch.
This leads me to assume that's why you can't do a return on Oyster without touching out. Because you'll only be charged £3.20.
These deductions also don't show up on the online journey history.