Need help with MODE_PACKING_8PPB monochrome 8 Pixels Per Byte mode

[martinberlin]

Hello all,
being a collaborator here longtime I feel a bit ashamed to ask about this. But we all miss some part of knowledge so it seems appropriate to start a new Discussion on the topic.
Specially for my LVGL experiments I would like to use epdiy with a fast Framebuffer that is WIDTH/8 * HEIGHT. That is a considerable reduction compared to the default 4PPB grayscale buffer specially when you do not want to use Grayscales and use your display to do only fast User Interfaces.
The problem is that is not documented and not really clear @vroland how to use the 8PPB mode. Here I did a fast example that is derivative from Dragon, just switching MODE_GC16 to MODE_PACKING_8PPB and trying to draw something over the dragon, directly memsetting the first 10 lines of the Framebuffer:
https://github.com/vroland/epdiy/tree/s3_color_implementation/examples/MODE_PACKING_8PPB

As a result the way it is now, using MODE_PACKING_8PPB is useless for me, or I'm missing something. Because with MODE_DU the 10 black lines come perfectly. But with 8PPB not at all, all display is white, can someone please point me in the right direction?