I think thats an issue with media. Basically the media looks too tight...so incorporating more perlite is a good idea. I don't know about this exact hybrid, but mirablis and tobaica are supposed to be easier species. And your temps don't sound bad. The way the plant looks is as if the roots are dying. I also personally don't like perlite in nep mixes as they can leech magnesium into the mix. But, I know others here use it without issues.
Another reason why this might happen is mites. Mites cause this typically to sumatran highland species. Same growth pattern.