See the commit messages and icons in those.
I don't know why but it seems the render-symbolic.py script (and hence gtk-encode-symbolic-svg) have some problems rendering correctly the image, although everything else I tried, including rsvg-convert, renders it correctly. Fixed.
Also, I just found out there was already an other tag icon in Builder's bundled icons, instead of the one I chose from Icon Library, . Any opinion on which one to choose ? Here's how it would look like: