Hi there,
I've got the same issue.
Mine is always changes back to 50Hz in config file.
I change it to 144Hz in the game, and when I exit the game it drops back to 50 in the file.
So I tried changing it to 144 in the file and it stayed like that for a while and then on some occasional game run it changed back to 50.
So I did something else, I changed it to 144 in the file, set the file to read-only, and it seems that it worked.
But I don't want to leave it like that, it is not supposed to be read only, am I right?
There is clearly a problem with that setting.