I am having the same issue with caption info showing up in the EXIF. I use only Photo Mechanic to write draft captions for images and upload them into an online archive system. When the captions are finalized and then later downloaded from the archive, the archive system overwrites any existing IPTC caption info with the most recent version. The archive system does not touch EXIF data.
However, when I view the image file properties in Windows or OSX, or upload into Fickr or Wordpress, the original caption that was written in Photo Mechanic appears in the description field, not the most recent caption that was written by the archive system into the IPTC data. If I view in PhotoMechanic or Photoshop the most recent IPTC caption appears.
It seems platforms that are looking at EXIF data are seeing the original Photo Mechanic caption. Platforms that read IPTC see the updated caption that was written by my archive system.
For some reason I am not able to attach a sample photo here, but would be happy to email.