This from one Steam forum post:
Reposting from another thread to save people some redownloading:
It’s a change on the Steam backend causing some oddly configured DLC depots to get dropped from the base app. This is not caused by the devs, so please do not blame them.
Redownloading will not help, because the information necessary to download and unlock the DLC files has temporarily been removed.
Other potentially affected games (check their SteamDB depots page for the “hasdepotsindlc” key at the bottom): https://steamdb.info/changelist/2494649/
For what it’s worth. Basically problems at Steam - probably working on it.