closing as wontfix as there's a simple workaround on this: use a truetype font.
the problem is once again due to the alpha channel. gd is setting it to 0 where it's written some text (which means opaque for itself) whereas it's writing to an image where 0 means completely transparent.
I probably should have dropped pixmap font support completely for agg, but as it works in non-RGBA mode I left it in instead of bailing out with an error message.
I think it's safe to consider that pixmap font support with agg is deprecated and is likely to be completely removed in future releases.