Hi,
A custom address field will only appear in the display of an address if its tag is added in the "address format" setting of the HikaShop configuration. Also, it's possible that after a migration from Joomla 3 to Joomla 4 (or changing your template), the field doesn't appear even though the tag is in that setting. That's because this setting will generate a view override of the default format in each template. And if you change to a new template, it won't have the override. So you'll have to save the setting once so that it regenerate the overrides.