Well after a few more experiments i can say it's definately the webgl renderer of cocoonjs that breaks everything...
here is my simple test project:
https://dl.dropboxusercontent.com/u/45711709/cocoonTest.capx
when webgl is enabled it looks like this on my phone (Sony Xperia Z):
<img src="http://i43.tinypic.com/4ucb9z.png" border="0">
when canvas is enabled it looks how it should:
<img src="http://i39.tinypic.com/s4s6dc.png" border="0">
overall a disappointing "update" :(
i was really looking forward to use webgl in my mobile games :(