You cannot change the field name once you’ve input content. You can change the field label freely. The content is being saved to that field name specifically… changing the field name is going to cause your previous content to disappear, because it’s been saved only to one field name.
However, you can retrieve this content by adding another field to the same field group, with the old name. For example, if you renamed a field from “title” to “title1”, your content will seem to disappear. In reality, it’s still saved in your database. If you add another field of the same type, and name it “title”, you will now see your old content still saved there.
You can change the field label without affecting the content saved. Or, if you really must change the field name, then create a second field with the new name, and just copy-paste into that field before removing the old one.
ACF Support Team here. This forum is generally used by ACF users to help each other out.
However, we would love to continue investigating and troubleshooting this issue, please can you create a ticket using our ?support form and we can look into it further.