CascadeTypeRemoveOrphan applies to lists. For that to work, you have to create a list of TUserTenant in your TUserAccount class:
[ManyValuedAssociation([TAssociationProp.Lazy], CascadeTypeAllRemoveOrphan, 'FUseraccount')]
property TenantUsers: TList<TTenantUser> read GetTenantUsers;
More info: Many to many with Aurelius