The Church, in the Council of Florence, says that to observe circuмcision, which means, to do or participate in cutting the male part, results in the loss of salvation, (unless one recovers, which means, confesses). Only mortal sin sends a person to hell. So why do you say it isn't mortally sinful?
Perhaps there is a bit of confusion over terminology. There are three conditions for mortal sin: grave matter, full knowledge, deliberate consent. One would consider all three of these if trying to figure out if a specific case involved mortal sin. But sometimes, when speaking in general, people say "mortal sin" to refer to objectively grave matter that can potentially be a mortal sin when the other conditions are met. For example, we often say that abortion is a mortal sin, using it in this latter sense. It is, however, theoretically possible for a specific instance of abortion not to be a mortal sin, as in the case of a woman forced to have an abortion against her will.
So, it might be more precise to say that observing circuмcision is objectively grave matter. It leads to loss of salvation when the other conditions for mortal sin are met.
(And this is made even more complicated due to the Novus Ordo tendency to push the knowledge and consent conditions to a point where practically nothing is a mortal sin. Obviously that is not right either.)