Sorry for my terrible english. I used the google translator.
I can add the text in German as an answer, if so desired.

Could it be that the truck has a problem with the cruise control?
There is no transmission with a retarder.
I use a manual shifter and the megashifter.
I apply the engine brake manually.
If I use cruise control and the truck gets faster and faster on its own downhill and exceeds the tolerance, the engine brake is activated automatically.
Question: Should that be the case? Not really. This behavior does not exist in the basic trucks from SCS, or the retarder switches on automatically, if at all.
BTW: I also saw this error with the Kenworth K100E here from the workshop, but not with the Freightliner FLB.

best regards
Dops