Vietnamese characters not displaying properly

I found this post: https://wordpress.org/support/topic/omgf-does-not-display-correctly-with-my-language/ while troubleshooting my posts.

On 5.8.3 (selected subsets Latin, Latin Extended), Vietnamese characters do not display correctly.

On 5.8.3 (selected subsets Latin, Latin Extended, Vietnamese), Vietnamese characters do not display correctly.

On 5.9.0 (selected subsets Latin, Latin Extended, Vietnamese), Vietnamese characters do not display correctly.

Only disabling the OMGF plugin displays it properly. I’ve left the plugin active for now so you can see.

The page I need help with: [log in to see the link]