It's good development to force Yae to think about romance, even if it's someone else's ex-romance. They aren't that young anymore, so I reckon for Yae it's less about genuine immaturity and more about wanting to keep playing the role of Zen's mother, since that's what she's used to. It's kind of pointless, though, to keep it going. It's a pretty remote possibility, but if Zen gave up and got a girlfriend from elsewhere, Yae obviously would also lose that role of hers. It would be the time for her to move on and accept being his love interest, unless she's actually opposed to that idea. In that case, they should become mere normal friends.