This is a question for HP or the HP support community, not for the Microsoft Community. See https://h30434.www3.hp.com/
That being said, this page explains what the lights on your printer mean: https://support.hp.com/us-en/document/c05920049
Also, the User Guide for the HP LaserJet Pro MFP M28-M31 is here: http://h10032.www1.hp.com/ctg/Manual/c05847830.pdf and there is a chart explaining the control panel light patterns starting on page 56.
What the basis for your understanding that the flashing light means "that the toner reservoir is full"? As far as I know, your printer does not have a "toner reservoir" or a "toner waste container." See, e.g., https://cartridgesdirect.com.au/blog/waste-toner-bottles excerpt below (emphasis added)
Do all laser printers have a waste toner bottle?
Although a waste toner bottle is extremely important to keep your printers performing at their very best, modern laser printers do not come with this component. Instead, modernised toner cartridges have a section engineered within the cartridge itself that will catch the toner for you. This means that every time you replace your toner cartridge, you will also be discarding the waste toner in the process. We may see the end of waste toner bottles in the future because of this technology. However, for now, they are still prevalent.
How do I know whether I have a waste toner bottle?
If you are unsure whether your printer needs a waste toner bottle, you can simply check the list of compatible printing supplies for your printer model to see if your machine uses one. If a waste toner bottle is not mentioned on the compatible parts list, then it is likely a part of the toner cartridge. Alternatively, you can refer to your printer manual to find out how your printer handles waste toner bottles. Remember, not all laser printers have waste toner bottle components.