[Plugin: WordPress SEO by Yoast] SEO score is not correct if keywords including unicode strings

The WP SEO by Yoast does not score correclty my posts with unicode strings. Please check this example:

Title:
Bán nhà ng? 466 ?ê La Thành, ??ng ?a - di?n tích 65.4m2, ng? r?ng
Content:
Bán nhà ng? 466 ?ê La Thành, ??ng ?a. Di?n tích 65.4m2. M?t ti?n 5.4m. H??ng Tay. Nhà 2 t?ng. ng? l?n, 2 oto có th? tránh nhau. Cách ???ng ?ê La Thành 70m
Keywords:
bán nhà,?ê la thành,??ng ?a

Result:

Your focus keyword was found in:
Article Heading: No
Page title: No
Page URL: No
Content: No
Meta description: No

Could you please fix the problems.

Please use following unicode equivalent for Vietnamese:

$coDau=array(“à”,”á”,”?”,”?”,”?”,”a”,”?”,”?”,”?”,”?”,”?”,”?”,”?”,”?”
,”?”,”?”,”?”,”è”,”é”,”?”,”?”,”?”,”ê”,”?”,”?”,”?”,”?”,”?”,”ì”,”í”,”?”,”?”,”?”,
“ò”,”ó”,”?”,”?”,”?”,”?”,”?”,”?”,”?”,”?”,”?”,”?”
,”?”,”?”,”?”,”?”,”?”,
“ù”,”ú”,”?”,”?”,”?”,”?”,”?”,”?”,”?”,”?”,”?”,
“?”,”y”,”?”,”?”,”?”,
“?”,
“à”,”á”,”?”,”?”,”?”,”?”,”?”,”?”,”?”,”?”,”?”,”?”
,”?”,”?”,”?”,”?”,”?”,
“è”,”é”,”?”,”?”,”?”,”ê”,”?”,”?”,”?”,”?”,”?”,
“ì”,”í”,”?”,”?”,”?”,
“ò”,”ó”,”?”,”?”,”?”,”?”,”?”,”?”,”?”,”?”,”?”,”?”
,”?”,”?”,”?”,”?”,”?”,
“ù”,”ú”,”?”,”?”,”?”,”?”,”?”,”?”,”?”,”?”,”?”,
“?”,”Y”,”?”,”?”,”?”,
“?”,”ê”,”ù”,”à”);

$khongDau=array(“a”,”a”,”a”,”a”,”a”,”a”,”a”,”a”,”a”,”a”,”a”
,”a”,”a”,”a”,”a”,”a”,”a”,
“e”,”e”,”e”,”e”,”e”,”e”,”e”,”e”,”e”,”e”,”e”,
“i”,”i”,”i”,”i”,”i”,
“o”,”o”,”o”,”o”,”o”,”o”,”o”,”o”,”o”,”o”,”o”,”o”
,”o”,”o”,”o”,”o”,”o”,
“u”,”u”,”u”,”u”,”u”,”u”,”u”,”u”,”u”,”u”,”u”,
“y”,”y”,”y”,”y”,”y”,
“d”,
“A”,”A”,”A”,”A”,”A”,”A”,”A”,”A”,”A”,”A”,”A”,”A”
,”A”,”A”,”A”,”A”,”A”,
“E”,”E”,”E”,”E”,”E”,”E”,”E”,”E”,”E”,”E”,”E”,
“I”,”I”,”I”,”I”,”I”,
“O”,”O”,”O”,”O”,”O”,”O”,”O”,”O”,”O”,”O”,”O”,”O”
,”O”,”O”,”O”,”O”,”O”,
“U”,”U”,”U”,”U”,”U”,”U”,”U”,”U”,”U”,”U”,”U”,
“Y”,”Y”,”Y”,”Y”,”Y”,
“D”,”e”,”u”,”a”);

Thanks,

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