blurry text HP Photosmart C4780

I am running Ubuntu 20.04 and HPLIP 3.20.3 I have an HP Photosmart C4780. I have genuine HP ink catridges. Black text prints blurry: there is a gray shadow of every character approximately 1mm below it which is made of a mosaic of coloured dots. If I switch the printing mode to "Black-only Grayscale" black text prints clearly. Colour photographs print clearly in colour mode. Text in any colour other than black prints fine in colour modes. But if I have a document to print that contains some colours anywhere and black text, I have no way to print it clearly. The printer's print Quality diagnostic report does NOT have the problem. A colour copy that includes clear black text is clear. The HPLIP testpage does have the problem.