It makes sense to me that Minsuk went back to his ex. They've been on and off for ten years, while he's only known Bexan for like a few weeks? Maybe months? People remember all the pretty memories and good times. Decide to see their ex one more time and then bam! They get hit with all the reasons why they broke up in the first place. I think it's pretty realistic.
I never refuted that? Even I know 10 years is a long time. My only complaint was how quickly it happened like two chapters which was a tad bit jarring. And again like you said he has known bexan for what...like a few months but running off to find him after immediately breaking things off with his ex (of 10 years) just didn't sit right with me....felt like a rebound.

"go to him, be with someone who can be with you wherever, whenever" bexan was giving minsuk a choice here and the latter actually ended up choosing his ex which is alright, not the problem, lingering feelings kinda stay with you for a long ass time (didnt they date for 10+ years? its believable) what annoys me is that literally next chapter, he realizes he shouldnt have given his ex another chance and the next chapter he tries finding bexan....like sure, fine. I have two problems with this.
1. super abrupt, should've given more than one chapter to break things off with his ex.
2. looking for bexan after dumping his ex (LITERALLY NEXT CHAPTER) is giving rebound even if the author didnt intend it that way
yknow what wouldve worked? not alluding that minsuk and the ex were thinking of getting back together. I would've preferred if it was an internal struggle between the ex and bexan in minsuk's mind instead of him jumping the gun and seeing his ex again (IN THE SPAN OF ONE FUCKING CHAPTER)