I always have to validate the integrity of the game files, which results in having to download the majority of the PTB twice. Why is this? Does anyone have a fix for it? This usually happens when I revert back to the regular build as well.
That's corrupt Steam download cache.
Or your anti-virus messing with the files, such as "cleaning" or "quarantining" them. Where in reality most anti-virus software just renames it or edits parts of the file. Whitelist your Steam folder from scanning.