>>280187970The point was to show how fucked up Phos was becoming. How he keeps trying to find something better and greater to "solve" everything. Phos promises Cinnabar he'll find him a better job, one where he won't have to isolate himself. Does this several times, almost rudely trying to get into Cinnabar's space over it. Tsundere that he is, Cinnabar rebuffs him but it's abundantly clear it's because he can't bring himself to hope that Phos is being sincere and will actually try, much less succeed.
Then we get the classic Zenkichi in Medaka Box thing where Cinnabar finally asks Phos about it, and it's clear Phos has long since forgotten about it while pursuing other plans. This fundamentally breaks Cinnabar, emotionally, when it comes to Phos. After this, despite becoming closer with the other gems, he will never think fondly of Phos again. Why would he? He never wanted to believe in Phos, but Phos kept pushing the issue until Cinnabar was genuinely hopeful about it, only to find out that, yeah, he never should've trusted Phos with that emotional connection.
Phos didn't mean to fuck it up, it's clear he does think going to the Mooninites will lead to helping Cinnabar, but it's become so gigantically detached from what Cinnabar actually wants and thought Phos was promising him that the relationship ruptures.