How to reproduce:
1. Import the attached "TextMeshProBug.unitypackage" Package to a new version
2. Open the "BugExample" Scene
3. Enter Play Mode
4. Click on the button at the center of the screen
5. Observe the textboxes

Expected Behavior: All text boxes are cleared since they are all linked
Actual Behavior: Only two first text boxes are cleared

Reproducible with(Unity versions): 2019.4.19f1, 2020.2.3f1, 2021.1.0b4, 2021.2.0a3
Reproducible with(Package versions): 2.1.1, 3.0.3

Could not test with: 2018.4 - the newest textmeshpro version there does not seem to work with linked text boxes

Note:
- Customer attached "Bug explain.pdf" for more details