Is a real estate profession that involves annually leasing apartments to non-Muslims who may reside in them with partners or in mixed-gender settings considered unlawful (haram)?

There is no harm in your renting out property to a non-Muslim for residence, even if he drinks alcohol or commits some forbidden acts. There is no sin upon the landlord because the rental contract was for a permissible purpose, and the sin is due to the tenant's actions. However, if you know that the tenant desires it for an illicit purpose, then it is not permissible for you to rent to him at that time.