Dear,
In version 1.0.3, I can use SF Mono is a fallback font to display Vietnamese characters. But in version >= 1.0.4, I can not do that anymore.
Version 1.0.3 no fallback font

Version 1.0.3 with fallback font

Version (1.0.4, 1.0.5) no fallback font

Version (1.0.4, 1.0.5) with fallback font

Thanks.
Yes, this happening because of added Vietnamese diacritics. Sorry, didn't know it will appear this way.
The workaround is to revert back to 1.0.3 & wait for the next update with support of Vietnamese language. As you can see it will be delivered soon. Link to the related request https://github.com/JetBrains/JetBrainsMono/issues/14
đã thấy thánh Hân ở đây rồi =))), đang định tạo issue
Thank @philippnurullin
@haipham22 resolved.

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Thank @philippnurullin
@haipham22 resolved.