does mastery grade consider time spent in combat?

I tend to play pretty passive enchanters and find that i can only get an S when getting many kills. Today i got an S- but i only went 1/2/19, i regularly do better than that and get only like A-, the main difference between this match and regular is that i spent a lot of time fighting enemies in combat. My damage wasnt notably higher than usual and kills was low.

I usually argue that kills are weighted too highly for supports but I recently also found out that healing/shielding allies doesnt place an enchanter into combat even when the ally is in combat so I'm wondering if perhaps the reason why it is so easy to get an S by doing damage vs by healing/shielding damage is actually because if the game uses time in combat as a stat then perhaps relatively passive enchanters arent being fairly accounted for since they spend so much time out of combat.