[Plugin; WordPress SEO by Yoast] Empty description

Hello,
i got an issue with empty description in some posts.
Here are some exemples of what is set in the excerpt field
‘ààààààà à’ => description is empty
‘ààààààààà’ => description contains the excerpt
‘ààààààà_à’ => description contains the excerpt
‘ééééééé é’ => description contains the excerpt

Is there an issue with some special chars and space character ?
Thank you