More important...if the hospitalization rate is significantly rising and the mandate (and presumably adherence to the mandate) didn't change...what did...? That's the relevant question.
I think that is answered by the Delta variant of the virus being so much more contagious than the prior version last year. Same reason we are seeing more breakthrough cases with the vaccines. All of the original testing and studies showing a 90-95% efficacy was comparing the original covid strain and not the new Delta variant. So now if vaccines are only say 65% effective at preventing the disease (just guesting here, I know there have been various numbers floating around), the same would seem to hold true for masks.
I'm vaccinated, my wife is vaccinated, but we've started wearing masks again and now its mandated again in Charlotte. But, it's not much of a surprise to me that masks would be less effective now than last year since Delta is so much more contagious. Still wearing a mask probably provides some benefit vs. not wearing one.