As you said they did not force that on Kaija it was her doing also this does not mean they can't hire not-so-close guardian to Jessica.
Anyway, it was not what I am trying to say. I think we are looking at Kaija&MC relation at a wrong perception. I think MC's parents are seeing Kaija as their daughter(not exactly but so close) as we know Kaija basically grew up in their house. She did not have good relations with her parents and only close relative of her, grandfather, died pretty early. If my assumption is correct then they probably did not want Kaija be part of Templars either but probably could not stop it, I assume it's related to Kaija's secret. So Grandmaster appointed Kaija as a guardian to MC not for MC's protection but Kaija's sake. He probably wanted to Kaija have a semblance of a normal life.
If so, there was no need to protect MC and Jessica therefore no protection for Jessica. Kaija was often lacking to protect MC, she was not around much to protect him either and these should be a problem for Templars because that put asset in danger but if there is no real danger then they easily overlook this. Also getting into relationship without authorisation would compromise the order but Templars did not say anything, which makes sense according to my theory because they see that, probably only Grand Master maybe Linnae too, as her own life rather than a duty.
Edit: I usually don't tag devs but
Ceolag , if it's not spoil any further plot point can you tell me, am I any close?