. 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.