just a quick note first to say that this thread is probably related to the discussion on another thread https://forum.inaturalist.org/t/observation-fields-not-allowing-edit-field-by-any-manager-or-creator/54789/7. that thread should probably be closed at this point, or else maybe it should be converted from a Bug Report to General thread and merged with ths one.
that said, it’s worth clarifying that the observation field data is not lost. the export is just returning data from a different observation field and not the original observation field.
i haven’t checked, but i assume when you export from the project page, you get a default set of fields returned in the export. before you actually generate the export, you should have an opportunity to select the exact fields you want to return in your CSV, or if you go from the Explore page, you should also be able to specify your own set of fields that you want to be returned in your export. so you should be able to select your old observation field, your new observation field, or both, to be returned.