Cycles is missing from the COMPAT_ENGINES in MATERIAL_PT_custom_props.

@Robert Guetzkow (rjg) technically Cycles is an addon, so we don't set it there, but in Cycles' own ui.py file. I had a look in there, and this panel is not excluded from the panel inclusion, so I don't actually understand why this doesn't work.

@William Reynish (billreynish) COMPAT_ENGINES in MATERIAL_PT_custom_props needs to include 'BLENDER_RENDER'. Cycles uses get_panels() (ui.py) and only adds the panel if 'BLENDER_RENDER' is in COMPAT_ENGINES and the panel is not in the list of excluded panels. This is also the recommended approach for third-party render engines, according to the API docs.

def get_panels():
    exclude_panels = {
        'DATA_PT_area',
        'DATA_PT_camera_dof',
        'DATA_PT_falloff_curve',
        'DATA_PT_light',
        'DATA_PT_preview',
        'DATA_PT_spot',
        'MATERIAL_PT_context_material',
        'MATERIAL_PT_preview',
        'NODE_DATA_PT_light',
        'NODE_DATA_PT_spot',
        'OBJECT_PT_visibility',
        'VIEWLAYER_PT_filter',
        'VIEWLAYER_PT_layer_passes',
        'RENDER_PT_post_processing',
        'RENDER_PT_simplify',
    }

    panels = []
    for panel in bpy.types.Panel.__subclasses__():
        if hasattr(panel, 'COMPAT_ENGINES') and 'BLENDER_RENDER' in panel.COMPAT_ENGINES:
            if panel.__name__ not in exclude_panels:
                panels.append(panel)

    return panels

You're right though, COMPAT_ENGINES shouldn't include 'CYCLES', since this would neither solve the problem for Cycles nor work for other external render engines. That was a misguided idea I had at the beginning. The patch uses 'BLENDER_RENDER', which is the correct solution.

I've already discussed this with @Brecht Van Lommel (brecht) on blender-coders, the patch works and has already been accepted by him.