"Is lost" is not the same as "lost." The boy (who lost at some contest) left the premises. The boy (who IS lost) left before the police arrived searching for him.
And typically the term "has" would be used rather than "is," in either of the two contexts. The lost boy has left. The boy who lost the contest has left.
As a "native" speaker, please PLEASE don' task me about why the past tense of "leave" is "left." In my heart I know it's not right, but if it's not right, it SHOULD be left, no?
Adriano Palma wrote:
Is the boy who lost left fails to have the following interpretation: (is it the case that) the boy who is lost left? Why does this string only mean: