The sentence is grammatically incorrect for the reasons you guess, and the Wikipedia chart confirms. If I were marking a standardized writing exam, I would mark this as an error.

The past perfect "*had learned*" makes it unambiguous that "must forget" has a past meaning, not a present one, so it's incorrect.

THAT SAID, in *unprepared speech*, rather than in writing, I could imagine a less educated native speaker saying this, and a lot of people not even noticing the mistake. I hear this type of mistake from sports broadcasters all the time.

So, it's not going to cause communication problems, but it is wrong.