Just to expand, even if it does turn out to be a race-based correlation (like we controlled for class, and the discrepancy was still present), we still don't know why.
One version is that a bunch of essentially similar cases come in, then the cases for white people are prioritized over the cases for PoC, because of racism. And therefore they get solved at a disproportionate rate. That's one potential explanation, that may have some potential solutions.
There are a bunch of different potential explanations though.
Maybe white people are more likely to be raped at parties, whereas PoC live in areas with less police enforcement so they are more likely to be raped by random strangers, so the crime is harder to solve.
Maybe black people are understandably less likely to talk to the police while they investigate anything, so it's harder to get witness testimony and the like.
Maybe there was a successful awareness effort targeted at black people, so they are now more likely to report when they are raped, whereas white people only report it when the case is especially egregious and egregious cases have more physical evidence.
I'm just making all these up to demonstrate that there are all kinds of potential explanations of why there's a difference between races even if the individual investigators try just as hard on these cases and the cases are assigned without bias. In basically all these cases the answer about how to improve this situation is different, so it's worth knowing the details. I think it deserves further study.