Check the adsets wherein the spike in CPM has been the highest using the Inspect tool and check for auction competition,it’ll give you a data trend if any to back up the hypothesis of rising CPMs

Post that,ideally,if you have a conversion objective try optimising for a lower than usual conversion event to target higher intent audiences. The logic herein being that CPMs are not in your direct control so present top funnel metrics are bound to rise. So you get more high intent audience than usual to reduce impact on your lower funnel cost metrics