Some of our ministries have seen reduce open rates since early October which may correlate to the last ACS upgrade. I have looked into the tracking log for some deliveries and I know there are missing opens. Since I receive many of the emails and open all of them and I don't have an open I know its missing. I still have the emails in my arch box and verified I read it.
Before I go through a long process with support I want to see if anyone else has noticed this.
Adobe support has found the issue and an update is needed to the instances options. It is isolated to GMAIL clients. I suggest if you are seeing this same issue log a support ticket and reference ticket #E-000259460.