The 5HP24 service manual has some details of the clutch operation logic so you can see which clutches are enabled in which gears etc. Might be worth starting there to see if you can see why its behaving like that.
Also, I noticed that in the S8 limp mode is 5th, rather than 4th which is normally cited - could be because the S8 gearing is lower? Limp is a mechanical feature though so not sure how the A8 and S8 could be different if that is the case.
There's a chance it could be the valve body. The Americans have a pressure valve mod that they do when the box starts misbehaving whereas we tend not to get involved in that sort of detail (throw it away or full rebuild). Might be worth reading up on that to see if it's related.
Are you able / inclined to pull the gearbox sump on the driveway? Shiny chunks in the sump is the ultimate diagnosis of a broken clutch drum. If there's nothing untoward in the sump it may be valve body related.
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