degortg - My first guess is the app cache is not refreshing so you dont see the changes. To test, change the background color to something else. If it stays black, then you know it has not updated. So you will need to cClear the cache or edit the manifest to resolve that.
Here are the steps I took to resolve it on my end:
1. I removed position:fixed from the css class
2. I wrapped the <canvas> tag in <center> tags
3. Tested in the XDK and it worked
4. Built for android and tested on my tablets and it worked.
Just an FYI this is not an issue with the appmobi build system or the appmobi plugin. I don't know what else I can do for you. What I recommended resolved the problem for me.
So if all the above does not work, I would recommend moving the question under a construct 2 general. It's a more appropriate forum for this question as it's not an appmobi or appmobi plugin issue. Also PM me if you do end up moving the question. I will do what I can to continue to assist you with this issue.
Best of luck!