Is anyone else seeing something similar?
Unless you believe that all of those people are using Apple, then it is unlikely to be that.
More likely you've turned tracking off, broken HTML, or Marketo is having an issue of some sort.
Apple‘s Mail Privacy Protection has the exact opposite effect. It doesn’t cause a dropoff in opens (i.e. pixel downloads). It causes every email to register an open, regardless of whether the end user ever saw it. So it’s 100% sure that isn’t causing a dropoff in opens.