Not a bug. ゝ is a hiragana repetition mark, similar to how 々 is a kanji repetition mark.
ゞ has dakuten indicating that this repeated character also gets dakuten i.e. the す becomes a ず
It's rarely used and supposed to only be used for proper nouns, e.g. company names like Isuzu.
However, there may be a forum bug in that it shows the hiragana version when I write this response, but displays the katakana version when I post it. But I'm not too worried about it since I don't think I'll likely ever use this symbol again.
