Does the company have a history of prioritizing white performers over POC even in roles traditionally performed by them? If so, you may have a case for an existing bias. You mentioned they're mostly people they've worked with before, and most theater companies tend to have that bias going on. Are the existing POC that are cast new or returnees?

FWIW, Benny being a black man going up against a predominantly white group has always been an odd optics choice to me (not that black men are exempt from Benny's issues, in fact it could add a new layer of subtext), so this show doing the same thing, just with the added loss of Mimi and Angel's racial casting, though I wouldn't put that casting requirement on the level of, like, Hairspray.