Generally, the property owner or manager is responsible for maintaining and repairing cluster mailboxes. This person or group (such as an HOA group) is in charge of performing necessary repairs to the mailbox. However, the local USPS office should maintain the USPS arrow locks and master door locks.
Note: The USPS maintains a small percentage of properties. However, the HOA or the individual managing the property would be aware of this scenario. Finally, a smaller percentage of properties may hold the tenant responsible for locks/keys. When renting/leasing, checking the rental agreement is usually a good idea.