To unlock the Relationship scene for Midnight you need to have Relationship Meter at 9.5, Morale at 8+ and complete their Urgent Job. Talking to them each Sunday in-game is possible to miss, and you need to talk to them twice on the first Sunday. Hope that helps!
Thanks for playing and glad you are enjoying your apprenticeship! If you have any more questions feel free to ask here or on our discord. (=
Cheers! -din