Will foundation be looked at at the same time, or is that a separate issue?
I see that on horizon and foundation the tag icons are not showing up under the topic title, but they are for default (and I think all the other themes available in the theme selector on meta). For illustration, note the fixed tag in the screenshots below:
Foundation and Horizon
Default
Also noticing while posting this that tag mentions are not getting the tag icon either. To test that, switch between default, foundation and horizon and see e.g. fixed has the icon on default but not on foundation or horizon.