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.