Confusing greyscale print problem

Printing from Adobe Illustrator to a "HP DesignJet Z2100 24-in Photo" only prints in grayscale.

Printing the same documents from other applications such as Affinity Designer, Adobe Viewer (Acrobat) and Photoshop prints in colour. I've adjusted the document colour type and any other things I can think of and nothing changes.

 

Furthermore, printing the same files from the same PC from AI to a Xerox Laser printer works perfectly normally. I can't determine if this is an AI issue or a printer issue, or what could be done about it in either case.

 

Much older versions of AI produce the same results.