My personal testing results:
- Regardless of theme I try on my phone, it is correctly aligned
- PC results:
2.1. Accessible Contrast dark:

2.2. Air Theme:

2.3. Central:

2.4. Dark:

2.5. Default:

2.6. Default (full-width):

2.7. Discourse Classic:

2.8. Fully:

2.9. Ghost:

2.10. Graceful:

2.11. Grey Amber:

2.12. Material Design:

2.13. Meta Branded:

2.14. Minima Dark:

2.15. Redditish:

2.16. Sam’s Simple Theme:

- If I adjust the width of the svg from 1em to 1.1em (manual CSS tweak), it looks centered on my PC:
