This is my own experience with this issue.
On my RTW itinerary, such DONE4, I flew many segments on QR with First name & Last Name ONLY on ticket. Never had a problem.
When flying on just QR issued tickets, during booking it asked for matching name with my passport. Couple of times during booking, I COULD NOT enter middle name (which my passport has) due to system "grayed out". I flew with that and not encountered any problem.
OT, I also encountered similar issues with CX. With or w/o middle name, I flew with no problem on CX.