A support ticket has been raised.
We don’t use the Advanced Custom Fields plugin so the functions.php edit for remove_wp_meta_box isn’t applicable in our case.
The Custom Fields section is enabled on the order details page and I can browse the dropdown list of custom fields available but the list does not contain billing_company_tax_id which I created in the Checkout Field Editor.
Other plugins like Abandoned Cart Pro use custom fields in our setup without issue.
HPOS is enabled; HPOS Data Caching is enabled; REDIS Object Cache is enabled.
