Not personal experience, but in working with mentally ill people, I will say that there are some people for whom medication does not entirely alleviate all symptoms.
For some people it may lessen the symptoms, or make them less severe to the point that the patient can function.
It could also be a matter of needing a medication adjustment or change or combination.
Psychiatric medications are an inexact science. It isn't lie antibiotics where you can culture an organism and determine what particular antibiotic the organism will respond to. With psychiatric medications it's trial and error and highly individualized. What works for one person may not work for another. It can be a long process finding out what medications/dosages/combinations work.
Yes, 17 yr old son with- bipolar had it when he was 12 through 14. Psychiatrist said it was called "breakthrough mania," because he was on Depakote & Abilify at the proper dosages, but still had mania about 3 evenings a week.
Some things I tried, which seemed to mitigate the mania:
Had him take his shower before dinner; it helped relax him so he wasn't as likely to get manic.
Earlier dinner, 5:45 instead of 6:30, because then he was able to take the evening medications earlier, too, which helped.
Tried to remind him about the bedtime book we were reading, to get him to focus on & look forward to getting ready for bed, which would keep him on track.
Almost any instance of him getting escalated in the evening would lead to him getting manic. Once he got manic, the evening was all about calming him down, which could take hours. Or he would get violent, which would lead to police, 5150s, hospitalizations, etc.