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 American sports broadcasters all the time (not Canadian or British ones).

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

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.