@lainfec Seinen and Shojo are demographics; young adult men and young girls respectively. Just cuz this story is a romance doesn’t disqualify it from being in a Seinen magazine. Like certain types of romance stories can be common for different demographics but I don’t really see anything out of the ordinary for a Seinen romance.
Hell if you look at Weekly Young Jump (Seinen), it has four romance stories.
Kaguya-sama
Kubo-san
Kowloon (by the author of this manga)
100 Girlfriends
Each one very different from one another and bring variety to a weekly magazine.