Normally I'd 100% agree with you, but i don't think what he was doing was necessarily stalking. When I think of stalking, I think of someone following someone around, peaking through windows, but isidor didn't do that, he simply made note of which paths he often saw her on and took them in hopes of crossing paths with her. Also, as the head of the information guild, he would ofc know things that may be creepy for other to know, but for him it's a job. Also being he buisness partner, he obviously knows what she's doing to some extent, without investigating or following her around, making it easy to cross paths with her. But he isn't following her with binoculars. Like if i had a crush on a guy who always ate lunch at a certain place, I might decide to have my lunch there in hopes of crossing paths with him. I wouldnt call that stalking. Idk, that's my opinion at least.
Like using your example, you had a crush on a guy but not only do you know his favorite place to eat, you know the routes he takes, he's schedule, all the clubs/classes he takes so you can take the same path and meet by "coincidence". While also watching him from a distance while he is at work. Then still wonder why he is suspicious about you.

Idk but the ML constantly stalking her and knowing her every move is kinda.... idk. Like I get some MLs be like that but to me it's not cute, it's kinda creepy. Like knowing her routine and stalking her is just too weird for me.