Looks like ''devil's contract'' which mika do with barbatos av system still affect him which make his right arm still crippled.
Thats why the season 2 Barbatos is going to be called "Barbatos Lupus." Lupus is a autoimmune disease the damages joints,skin and organs so you could say this is foreshadowing the second season.
Mikazuki Smile XD
He's smiling in s01 lineart too, the different is his crippled right arm and normal position for left arm