(This problem is actually a file system problem of OSX, here is just a scenario encountered when writing code for Xcode.)
This is an old question, from Xcode3.x to Xcode 14.x,
This broken problem has not been solved by Apple. If this problem occurs, it will not work if you force quit Xcode and reopen it.
The simple and crude solution is:
Restart the computer!
If it appears frequently, write notes in English!
As a friendly reminder, before restarting, you can save the code that cannot be saved temporarily in another text file.