Josh,
The photos were edited with Lightroom, but the captions still look fine on my laptop with Photomechanic5 after the edits. I've opened the same photos from the same thumbdrive on both systems and I'm having the same issue.
I've attached an image directly form my thumbdrive and one that has been copied from the older version of Photomechanic on my work PC respectively.
Do you think it could be an upgrade issue?
Not likely. I looked at the metadata on both images. In the XMP metadata none of the fields are truncated. In the IPTC metadata, the fields are truncated in the image named "090113PATRIOTphotos01.jpg".
So the reason you're not seeing truncation even though it is happening on some of your photos (probably because of some other application in your workflow like Lightroom) is because on your laptop you have it set to read XMP (the non-truncated data) before reading IPTC. The reason you're seeing truncated data on your other system is because it is set to read IPTC before XMP.
Adobe's applications have always enforced field length limits on IPTC (even though IPTC can handle fields of practically unlimited length) but not on XMP since the metadata standards differ between the two types. Photo Mechanic has never enforced those limits.
If you set both of your systems to read XMP before IPTC then you shouldn't ever see truncated fields. Now, if having the IPTC data never be truncated is important to you (IPTC is being phased out as a standard so maybe you don't care anymore) you'll have to make sure that if any of Adobe's tools update your files then you must update them one more time with PM via the IPTC Stationery Pad (just tell it to clear some always empty field) and PM will "fix" your IPTC data for you.
HTH,
-Kirk
