When pass specific characters are called Dakuon in Japanese language in msgstr to gtranslator, translation memory converts these characters to another characters which are not usually used.

How to reproduce ...
1. Open untranslated PO file
2. Input translated msgstr that contains Dakuon, for example "がぎぐげご"
3. Use above translated msgstr "がぎぐげご" from translation memory for another untranslated msgid
4. Save PO file
5. Search msgstr inputed step 2 by using another tool(e.g. grep or gedit search function).
6. It will return msgstr inputed step2, but not retrun msgstr inputed step3.

Regards