Don't worry, it does involved a third person, but nothing happened. And prospect of love triangle happening is eliminated quite fast even before it started. It also not causing any drama between MC and FMC.
IIRC, there's a male costar of Narumi who asked her to practising some scene from their movie/series, Narumi agreed then both of them go to some seaside walkway(? sorry don't remember). While doing that, some paparazzi caught them and make a big dating scandal about it. Because of that, Narumi had to stay with her manager for a quite some time, and Shun also going on vacation to the beach with his friend(to Okinawa i think), that's why they can't meet up for quite some time.
The dating scandal itself is more about how it's threatening Narumi's career as an actress, not the relationship between Shun and Narumi. The relationship between 2 of them is always fine (except when they missed each other all the times).
The male costar is actually interested with Narumi, it's just that Narumi always act professional with her costar/coworker and in the end, the chance of love triangle happening is over before it even started
And until the latest raw, this manga's still pure vanilla sweet romance and it's getting hotter and hotter each chapter. They even tried to have sex, but too bad it's failed
note:sorry for bad english