Yep, other people have had the same problem. I remember a discussion similar to this (Aged up a teen in Mortified mood swing, died by embarrassment after aging up).
With The Sims 4 Parenthood, teens have the strongest mood swings (that give 50 points). If they age up with uncontrolled emotions, they continue to get mood swings, but are much more weaker, only giving 2 points. In short, mood swings is not a childhood phase and happens randomly, after school or work, or after arguing. Because of this, teen who ages up with mood swing keeps the mood swing, and may die because the immunity of emotional deaths is removed.
Childhood phases are temporary traits like mean streak, picky eater, rebellious, and loud. There are also age exclusive childhood phases like I'm a Bear and clingy for children and distant for teens. As mood swings are just a moodlets that last 8 hours, it cannot be categorized as a phase.
Remember that and please be careful next time.
Tip. If you accidentally age up a teen with a mood swing, get a potion called Moodlet Solver. This is avalible in rewards store for 500 satisfaction points and is a dark green potion. Not only it removes negative moodlet, but it also maxes out needs.