Ok I see what happened here - thanks for the heads-up @merefield
Two things:
During some refactoring a few months ago, we inadvertantly started passing the ‘transformedPost’ instead of ‘post’ into post-bottom-ad
During the same refactoring, we left an old post-bottom/discourse-adplugin.hbs file lying around. It didn’t actually do anything, but it made this issue waaay more tricky to figure out