Brendan,
Let me start by first asking a couple of questions regarding those 1800 images:
1. Are you talking about NEF files here, or JPGs?
2. Have you set up the Colour Class names exactly the same in both PM and LR?
3. What is PM's read preference for IPTC/XMP? (Preferences, IPTC/XMP, "When reading IPTC/XMP")
4. Where do you have PM store your IPTC/XMP metadata? (Preferences, IPTC/XMP, "When writing IPTC/XMP")
5. Is lightroom updating the IPTX/XMP info to the .xmp sidecar files, or are they kept within its database?
For 1 I am assuming NEF. 2 should be YES. 3 should be something like "XMP sidecar", "Embedded XMP", "Embedded IPTC" for the TIFF based raw files. 4 the advised setting in your case is that all options are disabled for the TIFF based raw files (if you want, you can have the "Don't update even if exists" option turned-on in this case, though I do not really recommend this). 5 LR should be set to update the .xmp sidecar files, not only keep the info in its database.
With above settings all should be working. If not, please elaborate as to what is going wrong AND upload an example (raw file+.xmp sidecar file) as well as a screenshot of your PM XMP settings.
Hope this helps,