It does seem strange that you can't engine brake. How long do you continue to coast in 3rd gear at 90 in your example?
Does this only occur in 3rd gear and/or in a held gear and/or at higher speeds? For example, if you get up to 60 in 4th and then downshift to 3rd at 60, would you slow down or continue to coast?
ETA:
Possible your clutch in the DCT is slipping? Do you experience any issues with acceleration, i.e., going the other way? Would be interesting if the problem only happens in odd gears and not even gears.
I would search other BMW forums as well since many models have the DCT.
Two posts on other boards with possible similar issues:
https://f10.m5post.com/forums/showthread.php?t=1516509
https://f10.m5post.com/forums/showthread.php?t=1604851