Material assignments exhibit special behavior on linked AutoCAD Architecture objects and blocks, and the behavior is controlled by the Propagate Materials To Instances.

In the default state, assigning a material to any component of any linked AutoCAD Architecture object or block is equivalent to assigning the same material to every instance of that component of that object or block throughout your scene.

As an example, let's say you have multiple instances of a block named Telephone in your AutoCAD Architecture drawing, which consists of two nested blocks named Handset and Base. If you assign a material to one Handset anywhere in your scene, all the Handsets in all the Telephones throughout the scene will receive that material.

If you want to keep materials from propagating between instances in your scene, turn off the Propagate Materials To Instances.