Moritz's fine suggestion is the correct starting point for diagnosis, but I just wanted to suggest that this sounds like a file security problem.

When you modify and save a file, it likely changes the security attributes - owner, and permissions.

"Restoring" the file presumably keeps these attributes intact.

Suggestion: check these attributes ("ls -l" in linux, combination of "dir" and "cacls" or exploder GUI in windows) before and after your modifications.