Hi,
Try to swap the drums between them to see if the problem move to another color.
If this did not solve your issue it will be better to have a tech to replace the magenta developer housing and magenta developer material.
It could be also from the laser unit.
This was taken by reducing 50% colour of Magenta
Hi,
Please, upload some pictures so we can try to help you.
It is possible, from what have you described, to be from a dirty laser or from some debris inside the developer housing.
Gabi