Steam doesn't want to install the latest patch

[Massy_Pal]

So I've been playing Red Dragon without problems for 70 hours. I stopped playing for a bit, then reinstalled the game on an external hard drive for lack of space on my computer. After trying to play with some friends I discovered that I cannot seem to join their game, nor most other public lobbies, presumably because my game is stuck on v17 when the rest of them are currently on v21. After performing a clean install and verifying the files integrity, I cannot get Steam to install the actual v21 version as it infuriatingly keeps installing v17 for no apparent reason. If anyone has any lights to shed on the situation it'd be very appreciated.