Yes, there is lots of ambiguity, and some airlines may be more stringent (though even then there is lots of discretion on the agent’s part, e.g. the differing anecdotes for CX).

In this case, Qatar has decided to ask for ‘First name’ and ‘Last name’ specifically.

As per my previous example, my legal First Name is A, Middle Name is B, and Last Name is Z.
Given Name per my passport = “A B”
Surname per my passport = “Z”

As Qatar asks for First Name (and not Given Name), A specifically would be my appropriate response.