Ed ending as an adjective sounds differently
Solution 1:
The uses are different.
In the first case it is not the behaviour which did the learning; the behaviour was learned by someone.
Good table manners are a learned behaviour
In the second case it was the man who did the learning; he has learned something.
The physics professor is a learned man
It is not unreasonable for them to be pronounced differently, as they do not mean the same thing. This is called a heteronym, another example is attribute.