Adobe is not obligated to do anything that is not explicitly covered in the official documentation. Nor do they tell us their plans. That said, the actual functionality of any particular function or property often strays from the spec.
I've been at this for more than 20 years, in that time the compatibility of the field.buttonImportIcon() function has changed dramatically. Sometimes it works in Reader and sometimes it doesn't. You are much better off relying on the official documentation than the odd functionality that works outside the spec.
So no, there is no bug with this function in Reader because as I previously stated, it explicitly states in the reference that this function is not available in Reader.
However, there is one thing you could try. Convert your image to a PDF and use that as the imported button icon. PDFs do not require any image conversion. This has worked in the past in Reader.
Also try not including the "cPath" parameter, so that Reader displays a file open dialog. This bypasses the security issues.
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