I see these characters a lot in many of the books I read. They are used to
specify the beginning and end of a book.
From what I understand (I speak and read Khmer), these characters are not "missing" from LibreOffice. You can copy and paste them without any issue ៙ ៚ into LibreOffice and if you use a Khmer Unicode font, they will display correctly. And if you use a Khmer Keyboard Layout you can type these symbols (ALT-6 and ALT-7). So unless I misunderstood what you stated as the issue, this is not a bug.
Ah, sorry, I didn't realize this bug was for Dejavu...sorry! I thought all of these bugs were for LibreOffice...
I also agree having Khmer added to the fonts would be great - see also https://bugs.freedesktop.org/show_bug.cgi?id=11419