Well is adverb and known is adjective. As far as I know, it should not be hyphenated. For example: clearly impossible task, extremely powerful processor.
For starters, many words associated with well seems to be hyphenated.
well-written. Isn't this because of the fact that
powerful are all adjectives, known, fed, and written are not.
Well-known functions as an adjective as a whole, perhaps that is the reason why it is hyphenated.
This is a well-known person.
The use case here is that well-known was used as an adjective preceding a subject.
This person is well known.
The above statement works the other way.