I'm generating a pdf using the java api.
When setting embedding to false I get the following message
--> wrn: - |embedding font "AR PL ZenKai Uni, Medium"

It still ends up embedding the font into the pdf document.
Is this a bug?

This is not a bug: it means that you have used a character outside of the basic Latin range, and Prince has no option other than to embed the font (which is why it warns you that it is doing so).

Sorry I misunderstood then the embed functionality.
Thought it meant that the font file will be used only during
conversion and then not be embedded into the PDF document.

That is the idea yes, but at the moment characters outside the basic Latin range still require embedding the font. Support for font subsetting in the future will make embedding the font less costly, as only the glyphs that are actually used in the PDF will be embedded.

We have now added support for TrueType font subsetting to Prince 6.0 rev 4.