System Information
Operating system: Windows 10, Debian 10
Graphics card: Multiple

Blender Version
Broken: 2.91.0 alpha (#e414afbaf668550844451b1ca0c4c30541f9827d)
Worked: 2.81a

Short description of error
When iterating over materials and retrieving their default base color value, Blender returns false values.

Exact steps for others to reproduce the error

import bpy
for mat in bpy.data.materials:
    mat.use_nodes = True
    principled = next(n for n in mat.node_tree.nodes if n.type == 'BSDF_PRINCIPLED')
    base_color = principled.inputs['Base Color']
    value = base_color.default_value
    print(mat.name + ", " + str(value[0]) + ", " + str(value[1]) + ", " + str(value[2]) + "\n\n")

Blender 2.91.0 alpha prints the same value as r, g and b value - for every material. Example: 0.800000011920929, 0.800000011920929, 0.800000011920929

Blender 2.81a prints the correct values that differ from material to material. Example: 0.7465499639511108, 0.3384695053100586, 0.09304799139499664