[ctorbaoglu]

When playe a file on Kodi 19, it changes .srt files extension to .srt_backup and also it creates a copy of the .srt file and named as .srt_utf in the same directory. 
So it makes a problem for other media players so they cannot read the because changed extension of .srt files cannot be readable anymore. When I tried the rename the extension back to .srt (via deleting _backup at the end) but Kodi 19 rename the extension back to .srt_backup.

[Klojum]

but Kodi 19 rename the extension back to .srt_backup.

Kodi itself does not change filenames, perhaps your subtitle downloader add-on does that.
Start with providing your debug log (wiki) via a pastebin webpage, so we can see what is happening in Kodi.