Your system information

• Steam client version : 28 Oct 2020
• Distribution (e.g. Ubuntu): Ubuntu 20.04
• Opted into Steam client beta?: No
• Have you checked for system updates?: Yes

Please describe your issue in as much detail as possible:

Steam fails to detect existing shadercache when re-adding a steam library. The whole shadercache is re-downloaded (+background processing)

Steps for reproducing this issue:

1. Pre-requisite : having 2 Steam Libraries ( in my case : the small default on the SSD where Steam is installed + 1 Library on a big HDD)
2. For whatever reason, unmount the disk where the large Steam Library is located
3. Quit and relaunch Steam : all games from the large library are missing. (That is expected)
4. Mount the disk with the large library
   5 Quit and relaunch Steam. If necessary, add manually the large library. All games are detected.
   6 Steam does not detect the existing shadercache from the large library and starts to download-queue shaders for the whole library.

What is expected :

At step 5, I expect Steam to detect existing shadercache when he detects all installed games.