https://docs.mongodb.com/realm/sync/data-access-patterns/permissions/#sync-rules-and-permissions
One example of this is:
{ "%%user.id": [ "5f4863e4d49bd2191ff1e623", "5f48640dd49bd2191ff1e624", "5f486417d49bd2191ff1e625" ] }
Which should be:
{ "%%user.id": { "$in": [ "5f4863e4d49bd2191ff1e623", "5f48640dd49bd2191ff1e624", "5f486417d49bd2191ff1e625" ] } }