The file is saved as CMYK.
That makes sense. It's a pretty old HP printer so maybe it's just not cut out for it?
That could be the case. Even a lot of newer desktop and office printers can't interpret PDF-based effects very well. Sometimes updating the print driver can help. Printing a PDF directly from Adobe Acrobat might also solve some problems. But ultimately the printer and its driver is having to emulate certain kinds of effects. My workplace has a big office color laser printer for printing work orders, spreadsheets and other documents. It doesn't do so good at printing artwork from Illustrator. I'll see things like yucky discolored box syndrome on certain objects that have clipping masks and/or transparency effects. We have large format printers that output Illustrator artwork with no problems at all, but the key thing is the RIP applications used to feed data to those printers. They have Adobe certified PDF print engines in them. They understand all that PDF data natively.