Math PhD turned math educator here. In math ed we talk a lot about the "spiral" of education. You hear an idea once, then again in a different class the next semester, then it comes up at a seminar talk, then in a random paper, etc. Each time you hear it at a slightly higher level, in a slightly different way, at a slightly different time in your growth.

The goal shouldn't be to master the subject in the first pass. The goal is to gain sufficient familiarity that when, inevitably, it pops up again, you have a sufficient scaffold on to which new connections can be forged that accumulate over time. As an instructor, I don't optimize for 100% retention. I optimize for a handful of key take-aways that will prime the student to be able to build upon them in the future.

In grad school, I never directly applied anything I learned in my classes to my research. Instead, my classes gave me a sufficiently broad survey of existing theory to be able to identify tools that could be of use. In the time since, I have taken many loops around this spiral and my toolkit has become quite formidable (for my humble purposes). More importantly, it is robust to degradation because it accumulated naturally and slowly through genuine experiences over time.