Yes it could be something "technical" on LR side but the most probable cause is a mismatch between the label's names.
Contrary to the XMP Rating, which uses a well-defined numeric scale from 1 to 5, XML labels are just arbitrary text that sometime can be redefined by the user.
If someone renames the "Red" label to some other word or short sentence, this breaks the compatibility with all other XMP clients as other applications will not understand the new word and will likely ignore the value.
What would be needed are mapping tables in every app, to interpret various values written by every other apps, and perhaps this would at least partially solve the issue.
For a lack of a better solution, I find it's best to stick with the default LR convention ("Red", "Yellow", etc) as it maximizes the chances that other applications, including FPV, will read and interpret the data successfully.
FastPictureViewer Professional understands the default presets from a number of popular applications including Adobe Lightroom, IDimager, Nikon ViewNX and Microsoft ExpressionMedia, and can be configured in the program options to follow one or the other of those conventions.
Hopefully other XMP aware application also follow at least one of these conventions by default so there are good chances that a common ground can be found across two or three apps that a given user is susceptible to own, but again the user can ruin all this by renaming the labels, if only by starting to have inconsistent label values within his own picture collection.
The best is to pick one convention, say Lightroom's, and align all apps to it and never rename the labels.
The actual interpretation of the "color" should then be made "outside" of the system, for example on a Post It note that says Red=Publish, Yellow=For Review, etc.
Ideally, a mapping table in the applications would let the programs display "Publish" (or whatever) on-screen, but read/write "Red" in the underlying XMP data, i.e. provide one level of indirection which allows users to rename labels freely without breaking the underlying convention (just thinking out loud: this is what I'd do if I was writing a DAM...