Thank you for confirming this and updating it on this thread.
Just got a confirmation from the Target team saying this is a product bug and it is scoped for being fixed on September 30th.
Thanks for reaching out to Adobe Community.
There could be multiple factors causing this. You may want to try the below steps to troubleshoot:
1. To use mboxDebug, append an mboxDebug parameter to the end of your URL. The following table contains information about mbox-related URL parameters.
URL Parameters Purpose
Adding this parameter to any URL with mboxes defined opens a pop-up window with valuable debugging details. Cookie information, PCid and Session ID values are written out, and all of the mbox URLs are visible. Click on a mbox URL to show the response for that mbox. More details are available inmbox_debug.pdf.
Modify the cookie
Disable mboxes on the page
View profiles set.
Show response time for each mbox request
mboxOverride.browserIp=Insert IP address
Test geotargeting with this URL parameter. Type an IP address as the value for this attribute, and Test&Target's geotargeting evaluates that IP address to match against any geotargeting or segmentation set in a campaign.
2. Mbox.js sends a cookie called "em-disabled" to the visitor if target.js fails to load during delivery. This cookie prevents offers created using the Visual Experience Composer from rendering on the site. Visitors with this cookie neither see the test content nor get counted in those activity reports. All other offer content (from campaigns in Target Classic for example) continues to load. The cookie has a lifetime of 30 min from the time of load failure.
Also try other steps mentioned in the Troubleshooting Doc.