I don't think it's a duplicate in my case (unless behaviour has changed recently), as I do not normally experience the game redownloading when changing proton version. Also this is happening with the exact same proton version.
proton experimentalx1 2022-01
Your system information
When installing game for proton experimental (presumably any other proton version) via enabling "Steam Play for all other titles", if you switch to "Force the use of a specific Steam Play compatibility tool" and pick the exact same proton version, it redownload the entire game.
Steps for reproducing this issue: