Importing alembic -> assign material (shown in edit mode -> vanish in object mode)

I imported a self made alembic file (exported via Blender) and tried to assign different materials to it.

Works well - in edit mode.

If i change to object mode, Blender doesn’t show them anymore.

What’s happening?
Bug?
Did that ever happen to you?

edit object mode material gone

my alembic import settings: