Harlington no longer has Super Off-Peak fares to London. This is a deliberate change, performed as part of the fares 'simplification' exercise due to Harlington being one of the stations where contactless PAYG is being rolled out to tomorrow, under Phase 2a of
Project Oval.
Of course, as you have seen, this has unintended side effects such as making it cheaper to buy a ticket from further afield. In due course, contactless PAYG is expected to go as far north as Bedford, so you won't be able to avoid paying the Off-Peak fare at weekends anymore.
As part of the 'simplification' you can look forward to new evening peak restrictions on the Off-Peak fares. The only real benefit is that there is now single leg pricing, so there's no extra cost for returning the next day (or later).
The only place where you'll still find Off-Peak fares is if you nominate a destination beyond the PAYG area, e.g. Three Bridges. But a Super Off-Peak Day Return from Harlington to Three Bridges is more expensive than an Off-Peak Day Return to most places in London, and you can't get a Super Off-Peak Travelcard without resorting to the Flitwick alternative.