T9 Search issue on unicode characters

I'm Vietnamese, my contacts are saved using unicode character. Some of them can be searched using T9 keypad, some don't.

For example "Lâm" can be searched typing 526.

But "Đạt" can't be searched by typing 328.

Also in the image, typing 328 shows "Trung", but my contacts have 2 names "Dat", is this another issue?

This feature is there on android for 10 years and this is what Apple did?