Answer
At that point, yes — custom fields added specifically to a Master-Detail tab (rather than a header or overview screen) weren't surfacing their metadata under Page Configurations for ACP export, even though the fields themselves worked correctly on the page; this was confirmed as a genuine platform issue under R&D investigation rather than a configuration mistake. If exporting master-detail-tab custom field configuration via ACP is a hard requirement, check current release notes for whether this has since been resolved, since export/import via ACP for header and overview-level custom fields isn't affected by this same issue.