Dede,
At a first look both seems reasonable explanations, however, I cannot believe that the number of listings in Ipanema has multiplied from one month to another. Many of the listings that come up in a search for Ipanema (arguably Rio´s best neighborhood for visitors) are in Copacabana and other areas of the city, while our apartment is suddenly waaaay down on search page 15. This "downgrade" of our apartment does not make sense to me.
With respect to Zika, I don´t believe that is a big factor either. Firstly, the mosquito is mainly present during the summer months, when we were fully booked. Zika continues to be mentioned in the news because of the oncoming olympics, but I don´t belive it is affecting tourist flows more than in the summer months. Also because Ipanema is full of tourists still. Secondly, many of our guests (70%) are from Brazil and Argentina. This group would definitely not be particularly scared of the Zika virus now in June compared to when it was raging in November to February.
To me, there must be a technical issue on AirBnB´s side...