Hi!
Not sure if Im completely wrong with my workflow but Ill give you an example:
I have a piece of floor with material 1, and then Ive spawned some packed geos on top of it and they use material 2, Im using material node to assign the material 1 to the floor, so the packed geos are using the default material assigned on object level. then in the end Im merging all of this to one output. Im getting warnings in the merge node that some attributes may not be initialised correctly. and indeed, sometimes I have to go one node higher the merge and back to get the material assigments to work. they work nicely in viewport but mantra rendering isnt showig them properly sometimes

turning on the cryptic “overrides use local variables” checkbox in material node seems to help

that works like during the first render after tweaking the nodes, when I start another from the render view, materials arent correct

I found the ipr view doesn't like the mixed material applications when one has displacement shader enabled, and a material change requiring an update occurs. Simply hit the full re-render button the re-initialize the ipr session, or don't uses mixed materials that way. You can object merge out the geometry that had the displacement into a different geo node, and keep the the packed geometry inheriting the object material assignment in the first object. The problem doesn't really matter though because it only affects ipr updates. When you go to render your scene for real, it should render correctly. If you can, submit the scene with a bug description to Side Effects.

nice, the object_merge was new to me, works now nicely. thanks