Ah, good eye.
I thought the lodestone used avatars in a generic way, and it turns out that is not the case.
In order to generate the sigs we have to cache all the images used locally so as to not taint the canvas.
So my mis-assumption about avatars is what caused the inifini-cache to happen there, (what you described.)
I have purged it, and so you should be good to regen it with your latest loodestone avatar.
I'll look at purging those at whatever frequency they update on the lodestone side.
*goes and googles it*
Thanks a lot for noticing this! <3