The game window of S.T.A.L.K.E.R: Shadow of Chernobyl (4500) can't get moved to secondary monitor. Once moving it to the second monitor and entering the full screen game again, it jumps back to primary monitor. Other games running with Proton can do this.
Which games were you able to successfully do this with? I am seeing only failure like you described when trying to play a game full-screen on a second monitor.
The game window of S.T.A.L.K.E.R: Shadow of Chernobyl (4500) can't get moved to secondary monitor. Once moving it to the second monitor and entering the full screen game again, it jumps back to primary monitor. Other games running with Proton can do this.
System info: https://gist.github.com/MaxXor/501c9c442b3ecb4a125f00038845b532