We disable all relation population in /users/me unless you extend the users-permissions plugin, specifically this service:
This is done for performance reasons, you can see in that code sample that only role exists in the population array, just add the field name of the relation field you wish to populate. If you need to go deeper you can do something like:
['role', 'orders', 'orders.someNestedRelation']
See: Strapi plugins - Strapi Developer Documentation to see how to extend a plugin.