He didn't really raise her, he simply dropped her off to be raised as a priestess and went on his way.
Knowing her since a child and then when they're grown and fucking them... Still in the same realm. He and she spent quality time together until he dropped her off. So much so that she developed feelings from him and had flashback memories of the different things they did when she was a CHILD. So it's the "I raised you and now imma fuck you" troupe.
I can see why it's adjacent to that trope, but I don't think it's as bad as the way you're framing it. He didn't plan on ever seeing her again, since he thought that she would hate him for being a mage due to being in the temple for so long. He never indicated to her that she should become a sharing when she gets older, so it's not like he groomed her.
It would be weird if he specifically brought her into the tower to become a sharing, but that's not what happened. They separated for years and then she pursued him as an adult woman, which I don't see anything wrong with.
I will say, I would have liked more development between them since their relationship is basically a lust=love situation, on account of poor writing from the author. You're right in saying that the way he immediately became attracted to her is odd, but I think that's more so related to the fact that there was no relationship growth to establish that change. If there was more time where he got to know the adult her and fall in love, then I think that would really benefit the relationship overall.
The mage didn't raise her tho?
Why yet again is this the troupe of "I raised you and now imma fuck you"? Like these authors are very weird for that. The stories in itself be decent but then the backstory of the having raised their now current love interest is gross AF.