I know everyone hates this plot, and I can't say I love it, but I do feel like it's pretty in-character for Tomoka to be a huge dumbass who bets her future on a race without thinking about the implications. Like, Tomoka genuinely loves Yui and is an overall good person, but she she's never been the most emotionally intelligent or perceptive. I totally buy that that she'd miss Fukami's obvious crush on her (one that Fukami herself doesn't realize, and thus is sublimating it into this weird and inappropriate jealousy) and not realize what's really got her roommate so possessive.
She's also very susceptible to peer-pressure and tends to think of everything in terms of an athletic competition, so I feel like of course she jumps on that - a simple race - as a way out of this uncomfortable social situation. And it's not until afterward that she realizes just how much she's disrupting the plans she and Yui were making. And Yui is too nice and too passive to call her out over it, so rather than pushing back she kind of just accepts it. (Yui's historically shown a reluctance to "interfere" in Tomoka's athletics, as well)
Anyway, I'm willing to see where this goes. This isn't the first time there's been some frustrating drama, but I feel like it almost always pulls it back, shows how the behavior was an issue and then has the two work through things to be in a better place. Like, funnily enough, the last time Fukami caused Yui grief when they were on the summer trip; Fukami was weirdly possessive, Yui was insecure...but, ultimately, it led to my favorite chapter of either run - Crickets and Kase-san - where Tomoka makes crystal clear that she sees her future as being with Yui ("I'll go where you go"), and Yui lets go of her insecurities all on her own. (instead of relying on Tomoka to reassure her, though bringing up Fukami's actions there might have saved some grief this time around...)