Published-Draft articles are not searchable
is this a known issue with the search or it is the expected behaviour?


It’s very much the expected behaviour


  If yes, any specific reason for implementing like this?


Yes. Only published content should be visible, unpublished content should not. That’s the whole point of the distinction between the two states.

If unpublished content appeared in search results (without manual intervention if there was an edge-case requirement  to do so), that would be a major bug.