Is the prayer of one who performed tayammum and prayed Maghrib valid because of a water قطع (cut-off) from his apartment and building only, or is he required to repeat it?

If the situation is as you described, with water available in areas adjacent to your building, then your prayer is invalid, and you must make it up. This is because a condition for the validity of prayer with tayammum (definition) (dry ablution) is that it must be performed after seeking and searching for water. Allah Almighty says: (and you find no water, then perform tayammum with clean earth).