@sanfordwhiteman , thanks for the hat tip.
@camrynka , Adobe have moved the tracking pixel from the bottom of the email to the top of the email. The one thing that precedes the tracking pixel? The BUILT IN PREHEADER. If you are using a hand rolled preheader it will appear after the tracking pixel. If you use no preheader, then the tracking pixel comes before everything else. The tracking pixel link (as you're aware) then displays in Outlook.
In so doing, Adobe have done one thing smart. Moving the tracking pixel to the top of the email will boost open rates on long emails sent to Gmail (because Gmail doesn't load the entire email, anyone who opened the email but did not tell google to display the whole message would not be marked as as an open). And, in all fairness, the issue you're experiencing is actually a bug in Outlook. Why Outlook is displaying an image link as a preheader I have no idea. BUT, Adobe should have tested this much more thoroughly, and so the fault ultimately lies with them.
I tried all kinds of ways to work around this for hand rolled preheaders (mimicking the class, the id etc). However, nothing I did worked.
Cheers
Jo