According to the text of Mantle of Majesty (an article by Bard College of the College of Glamor, Xanathar Guide for Everything, page 14):
As a bonus action, you cast Command, without spending a spell slot, and you acquire an appearance of supernatural beauty for 1 minute or until your concentration ends (as if you were concentrating on a spell).
Specifically, the "as if you're concentrating on a spell" part bothers me. According to the rules, the things that can make you lose concentration are:
Cast another spell that requires Concentration. You lose Concentration in a spell if you cast another spell that requires Concentration. You can not focus on two spells at a time.
Taking the damage. Whenever you take damage while concentrating on a spell, you must make a Constitution saving throw to maintain your concentration. The CD is equal to 10 or half of the damage you receive, whichever is greater. If you receive damage from multiple sources, such as an arrow and the breath of a dragon, you make a save roll separately for each source of damage.
Be incapacitated or killed You lose concentration in a spell if you are incapacitated or if you die.
Class characteristics are not spells. So, does this mean that if you cast a concentration spell and then use the Majesty's Mantle, you can concentrate on both of them as long as you do not take damage?