MC is better off breaking up with this girl imo. She’s selfish, doesn’t tell a shit about her work and then disappearing all of a sudden lol. Might as well focus on your work and leave the girlfriend role to the childhood friend.
It's not the first time she just disappeared for job abroad.
I always feel like Hiyori is desiring a piece of normality, being just a teenage girl in love, living peaceful daily life. That's why she tried to separate that side of her life as much as possible from Miharu.
Also there's always the risk of information leaking out if too much people are in the know.
And yeah, she's seeing Eri as a challenge. In volume 2,
Hiyori is really feeling inferior to Eri. She saw how both are very close. Eri is also very beautiful compared to her (having stressful job going around the world is not good for beauty). Hiyori even personally think of her as "the heroine of this story", someone who deserve being with Miharu more than her (and her power went to ask Eri and Miharu to have that feeling for each other).