This was mentioned by @MikeWh in the Project Oval thread, but I would have missed it if it wasn't pointed out to me by someone else, so I thought I'd also mention it in here in case others wanted to discuss it further.
From the post on OysterFares.com:
There has been a development. https://oysterfares.com/all-oyster-cards-now-re-calculate-overnight/
This means that the back office now understands discounted fares, which has probably been one of the stumbling blocks.
From the post on OysterFares.com:
A few years ago TfL introduced retrospective weekly capping for blue adult Oyster cards without any discounts. This meant that each days journeys were run through the same back-office function as is used to calculate daily and weekly charges for contactless cards and devices. At the time it was stated that discounted Oyster cards would follow, but that seemed to be taking forever. Until now.
It actually started on December 7th 2025. I noticed an unexplained refund when looking at someones journey history and asked the question. I then tried it myself and lo and behold a refund came back. I’m not quite sure why they haven’t sought to publicise this more widely, but here it is.
So, what happens? Each day the journey touches are processed through the back office. This calculates the best combination of zonal caps and extension fares and arrives at a total. If that is less than the amount actually charged by the Oyster system then an adjustment is put in a queue. At the end of the week it also works out the best combination of weekly cap and extension fares and again compares that to what has been charged throughout the Monday to Sunday week. The adjustments are totalled each night and if they exceed £1.50 a credit is put on the Faster Universal Load (FUL) system to be picked up next time the card is touched on a reader as part of a journey. Two weeks after the first un-processed adjustment is raised any amount outstanding is added to FUL.