How to reproduce (simplified repro steps, see the original user report in the Comments for the original steps):
1. Open a new Scene
2. Create a Cube in the hierarchy
3. Attach "MaterialTestScript.cs" to the Cube
4. Create a Material asset in the project browser and name it "TestMaterial"
5. Drag the Material asset to the Material field of “Material Test” Script Component on the Cube
6. Enter Play Mode
7. Open the Settings of the "Material Test" Script Component in the Inspector (Three dots)
8. Press "Set Material"
9. Notice how the Material on the Renderer gets set to "TestMaterial"
10. Stay in Play Mode and again open the Settings of the "Material Test" Script Component
11. Press "Get Material"

Expected result: The Material on the Renderer does not change, it stays "TestMaterial"
Actual result: The Material on the Renderer changes to "TestMaterial (instance)"

Reproducible with: 2021.3.12f1, 2022.1.21f1, 2022.2.0b13, 2023.1.0a17, 2023.2, 2023.3
Couldn’t test with: 2020.3.41f1 (Script errors: “Tuple must contain at least two elements”)

Reproducible on: macOS Monterey 12.6 (Intel), Windows 10