Hello,
Your changes won’t be lost if you modify custom.css file only and not original style.css. If you’re using custom.css of the parent theme, that’s fine. Custom CSS file is used to overwrite original css styles. It’s not recommended to edit original style.css.
You can make changes either in custom.css file or in child style.css.
Only if you made changes in template files instead of child theme then these changes will be lost. That’s why we always recommend to use child theme for all the types of customization.
Regards,
Rose Tyler.