I've found that a few Vietnamese diacritical marks are broken in a certain condition. See the image below:

There are 2 diacritical marks that broke:
- dấu nặng (dot)
- dấu ngã (tilde)
A funny thing is while it generally broke every where but in Preview in Paragraph, both italic & non italic are broken. Below 16px size are also broken in Preview in Sentence but not Paragraph.
More info for reproducing
- OS: Linux / Ubuntu-based Tuxedo 2
- Firefox: test 2 time with cache cleared, test 1 time in a new profile → all failed
- Chrome: work just fine