I have had several printers were the cleaning cartridge has run out and therefore causing the wiper not to be wet which has caused head issues. Really need to see the nozzle test print and of course the service level nozzle test print which would really show the issue if you have them with a head.
please follow Jim‘s advice or just print in unidirectional and see if the problem goes away. If the problem goes away when printing in unidirectional, then you know you need to form the M gap adjustment.