This depends on the PDF version. As far as I know, PDF up to at least 1.4 didn't allow anything else than 3 components in the /C entry of an annotation. So a CMYK PDF/A-1 document, for example, cannot have colored annotation borders. This has changed as per PDF 1.7 anyway, where annotation borders can be specified as other color arrays, hence four components for a DeviceCMYK color.
I know you are refering to LaTeX Hyperref. More than a bug in Adobe products, this is a weird behavior of the package. There are other ways (specifically an appearance dictionary) to decorate an annotation (even with a colored border) than using the /C entry (which originally was intended for display only).
This is not a bug.
This depends on the PDF version. As far as I know, PDF up to at least 1.4 didn't allow anything else than 3 components in the /C entry of an annotation. So a CMYK PDF/A-1 document, for example, cannot have colored annotation borders. This has changed as per PDF 1.7 anyway, where annotation borders can be specified as other color arrays, hence four components for a DeviceCMYK color.
I know you are refering to LaTeX Hyperref. More than a bug in Adobe products, this is a weird behavior of the package. There are other ways (specifically an appearance dictionary) to decorate an annotation (even with a colored border) than using the /C entry (which originally was intended for display only).
Extract : PDF 1.4 specification, p. 491.
Hope this helps! Boris Oriet