Reproduction steps:
1. Open the attached project "ReproProj"
2. Open the “/Assets/Scenes/SampleScene.unity” Scene
3. Enter the Play Mode
4. Click on the blue dot furthest to the right
5. Click on the green dot furthest to the right
6 Click on the blue dot furthest to the left
7. Click on the green dot furthest to the right
8. Click on the blue dot furthest to the right
9. Click on the green dot furthest to the left
10. Drag the hamburger handles around

Expected result: Only the splines that are connected to the moved dots changes
Actual result: All splines are moving

Reproducible with: 2.6.1 (6000.0.29f1), 2.7.2 (2022.3.53f1, 6000.0.29f1, 6000.1.0a6)

Reproducible on:
Play Mode
Windows Standalone Player

Testing environment: Windows 10 Enterprise 21H2
Not reproducible on: No other environment tested

Note:
- Forum thread: https://discussions.unity.com/t/spline-extrude-on-prefab-object-is-somehow-sharing-data-from-all-instances/1489381