I've had a bad batch of black toner months ago. Would cause a random fine black streak that would go edge to edge in the process direction for an also random amount of sheets.
Took about a week to solve after replacing black dev a few times and actually putting in a whole new unit....
Then my techs get the email saying "batch so and so has been contaminated".
They sent me batch so and so.
Problem was solved when the toner wasn't contaminated lol. Weird how that works.