Reproduction steps:
1. Open the attached project "ReproProj"
2. Open the “/Assets/TestScene.unity” Scene
3. Enter the Play Mode
4. Observe the 2nd line of text in the Game view

Expected result: The line does not jitter
Actual result: Line is jittering

Reproducible with: 6000.0.40f1, 6000.1.0b8, 6000.2.0a5
Could not test with 2022.3.59f1 (Editor crashes after installing TextMeshPro package)

Reproducible on:
Play Mode
Windows Standalone Player

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

Notes:
- Jittering is caused by changing the last character in the line
- Text is right aligned and in Horizontal Layout Group
- Text is using “Mono” style which implements the mspace rich text tag
- Forum thread: https://discussions.unity.com/t/monospacing-option-on-inspector/753661/5