I don't know how the JoinProcessor works exactly.
Anyway, following this answer this answer I actually found out that Resource Models belonging to magento-customer inject a dependency on JoinProcessor while those belonging to magento-sales don't. I think this should explain why the attribute is working in the latter case and not in the former: in this case an explicit data loading is required (please have a look to Gift Message case). This can be done for example by using Plugins.
I don't know how the JoinProcessor works exactly.
Anyway, following this answer I actually found out that Resource Models belonging to magento-customer inject a dependency on JoinProcessor while those belonging to magento-sales don't. I think this should explain why the attribute is working in the latter case and not in the former: in this case an explicit data loading is required (please have a look to Gift Message case). This can be done for example by using Plugins.
I don't know how the JoinProcessor works exactly.
Anyway, following this answer I actually found out that Resource Models belonging to magento-customer inject a dependency on JoinProcessor while those belonging to magento-sales don't. I think this should explain why the attribute is working in the latter case and not in the former: in this case an explicit data loading is required (please have a look to Gift Message case). This can be done for example by using Plugins.
I don't know how the JoinProcessor works exactly.
Anyway, following this answer I actually found out that Resource Models belonging to magento-customer inject a dependency on JoinProcessor while those belonging to magento-sales don't. I think this should explain why the attribute is working in the latter case and not in the former: in this case an explicit data loading is required (please have a look to Gift Message case). This can be done for example by using Plugins.