Your system information

• Steam client version: Dec 8, 2018
• Distribution: Arch Linux
• Opted into Steam client beta?: Yes
• Have you checked for system updates?: Yes

Please describe your issue in as much detail as possible:

Expected:
When buying the full game after playing a demo, Steam should download missing files and keep existing files if the game needs them.
What did happen:
Buying the full game after playing a demo, Steam downloads missing files (~2.5 GB) and deletes existing (~30 GB). Verifying integrity of game files forces it to download missing files.

Steps for reproducing this issue:

1. Install and play The Evil Within 2 Demo (via Proton)
2. Exit the game and buy it.
3. "TheEvilWithin2" folder gets deleted from _library_/steamapps/common/ , Steam downloads ~2.5GB.
4. "TheEvilWithin2" folder in _library_/steamapps/common/ is ~2.5GB and it shows up as full game downloaded and installed.
5. Verifying integrity of game files will fix the issue by downloading deleted files again.