Actually remembered something, so I'd thought I would try it again, everytime I put a fresh install of ACC I get a 21.9mb update come through for Raceroom, that's abit odd, I've done it now 3 times and the same result everytime.
Well that's fun, can someone check what's being downloaded? While the download is progressing you can check in this folder what exactly is being fetched: ...\SteamLibrary\steamapps\downloading\...
Well I had ago this morning, the problem for me is I'm on a very quick connection, so with the RR download it happens really quick, but I can see what belongs to ACC, and the only other thing that appears is another folder which is labelled 228980 can't tell you if anything was inside it as it happens so fast it vanishes, so unable to click on it.
Nope, I can't pause it fast enough, it happens so quick, also when uninstalling another download for RR appears which is 0mb.
For your reference, Process Monitor would be helpful. ( https://docs.microsoft.com/en-us/sysinternals/downloads/procmon ) You can filter to monitor the file I/O like this. Here is an example of verifying R3E integrity check.