If a panel field is set to translate:false, the non-default value will be deleted on saving the default-language page. (Solving Translate:false and save)
If the URL-Key is set in the non-default version by using the page url editor, the deletion fails and can only be fixed by switching to the non-default language and clicking the save button. (at least there is a workaround)
Where does the original field value in the non-default language come from if the field is set to translate:false?
Could you create an issue on GitHub in the Panel repo?
All values of the default language content will be saved to the non-default language content txt if you change the url-appendix of the non-default language page version.
Issue created https://github.com/getkirby/panel/issues/1056