Jorrit, I checked your query and seems that the polygon is being interpreted a bit different in your prototype, you can see an example of your query at this url:
http://www.gbif.org/occurrence/search?GEOMETRY=116.71875%2055.7765730186677,116.71875%2090,87.1875%2090,87.1875%2055.7765730186677,116.71875%2055.7765730186677
which is equivalent to this API call:
http://api.gbif.org/v1/occurrence/search?GEOMETRY=POLYGON%28%28116.71875%2055.7765730186677,116.71875%2090,87.1875%2090,87.1875%2055.7765730186677,116.71875%2055.7765730186677%29%29

If you checked the first link you can see that the polygon looks different to the polygon in your prototype, can you validate that your prototype is drawing the polygon correctly? 

Answering your questions:
1. Dateline crossing is supported by the search api



PS1 Is there a github issue list than I can post this at?
2. Yes, you can use the “feedback” (right side of the screen) in the gif portal http://www.gbif.org/  or you can sign-up to our JIRA http://dev.gbif.org/issues/login.jsp

Federico Mendez
GBIF-Developer


PS2 For context, please see a working prototype at: http://www.effechecka.org/#geometry=POLYGON%28%28116.71875%2055.7765730186677%2C116.71875%2090%2C87.1875%2090%2C87.1875%2055.7765730186677%2C116.71875%2055.7765730186677%29%29&hasSpatialIssue=false&height=202&lat=79.81230226556366&limit=20&lng=-78.046875&scientificName=Aves%2CMammalia%2CMammalia&taxonSelector=Aves%2CMammalia%2CMammalia&traitSelector=&width=470&wktString=ENVELOPE%28116.71875%2C87.1875%2C90%2C55.7765730186677%29&zoom=1