[bug] It's a bug?

In this analysis
http://ru.lichess.org/P0XTF78U/black#0
In comment for 15.Qg2!? there is the impossible moves:
19.... Rg8 and 20.... 0-0-0
P.S. Often I saw that a position evaluation causeless jumps to 0 by repeating of some position.
Thanks.

Hmm... At now already another rook moves to g8 19.... Rg8. All is correct. Topic can be deleted. Thanks.