Good Day. A warm welcome to the HP community. I reviewed the case regarding issues with not being able to print in black or grayscale as it does not retain them. Superb description, brilliant troubleshooting and terrific observations made before posting. Kudos to you for that. 
To assist you with accurate information I would require more information regarding this:
- Did you run HP print and scan doctor and check if it gets fixed?
For now, try these steps:
Please perform a clean uninstallation and reinstallation of the printer drivers by following these steps:
- In Windows, search for and open Programs and Features.
- Select your HP printer.
- Select Uninstall.
- In Windows, search for and open Devices and Printers.
- In the Devices and Printers window look for your HP Printer. If you see it listed, right-click on it and choose "Delete" or "Remove Device.”
- Open up the run command with the "Windows Key + R" key combo.
- Type printui.exe /s and click OK.
- Click on Drivers tab.
- Look for HP Printer driver. If you see it click on it and click Remove at the bottom.
- Select OK.
- Remove all instances of the same.
- Select Apply and OK on the Print Server Properties windows.
- Close Devices and Printers.
- Restart the PC again.
Then download the latest full feature driver from https://hp.care/2sRmB5a and install it on the computer after selecting the correct operating system.
Connect the printer to your computer only when the setup prompts for the printer.
This should fix the issue.
If the issue continues, then please download the Windows Built-in drivers by following instructions from this link: https://hp.care/2o3acvH
This will resolve the issue for you.
- Now for further assistance, please refer to this document: https://hp.care/2I1Z09Z (HP Inkjet Printers - Printing with Black Ink Only (Windows) Refer to the FAQs
- Also check this printer settings link: https://hp.care/2I08RwY (HP Printers - Print Driver Settings Guide (Windows) refer to the section (Color and black & white settings)
This should do the trick for you.
Good Luck and keep me posted. I wish you well. I will keep a watch for your reply.
Please perform all these steps patiently as it is critical to resolving the issue. Going by your technical expertise and competence I am sure you will be able to perform these steps without breaking a sweat.
Hope this helps. Let me know how this goes. I genuinely hope the issue gets resolved without hassles and the unit works great.Please reach out for any issues and I'll be there to assist you.
To simply say thanks, please click the "Thumbs Up" button to give me a Kudos to appreciate my efforts to help.
If this helps, please mark this as “Accepted Solution” as it will help several others with the same issue to get it resolved without hassles.
Take care now and have a splendid week ahead.