I have 2 K850's that started playing up with the magenta printing just after I read this thread so I think there is some voodoo going on.
I've stuck a blunt hypo into the ink feed tubes and can quite easily draw the magenta from a newly inserted fake hp cartridge.
I've tried 3 print heads from other printers and they all show naff all magenta coming through on the print head.
The printer also prints out a configuration page after every print you manage to get through it I presume because it doesn't think the heads are aligned correctly?
I've been using the same refilled cartridges for months.