>   . did you put the text property only on the single character SHIN or
>     on the whole Hebrew text?
>
 Only on single character using (put-text-property (point) (1+ (point))
'face '(:weight bold :slant oblique)) while the point was under that
character.

  . what does describe-char report for the SHIN character before you
>     put the text property?
>
Linux Libertine Display O


> . Your original report shows Hebrew text with diacritics (so-called
>     "Hebrew points").  Specifically, the letter SHIN is followed by
>     two such diacritics: U+05C1 and U+05B8.  Emacs composes those on
>     display into a single grapheme cluster.  If you use the Hebrew
>     text without those diacritics, i.e. just "שלום", does the problem
>     happen in that case as well, i.e. do you still see the incorrect
>     font name after adding the text property?
>
 This atleast applies the weight correctly but reported font is DejaVu
Sans-bold.