I have two almost identical buttons, but the icon is only showing for one of them:

1. /cast [@focus] Moonfire
2. /cast [@focus] Sunfire
   The icon only shows for moonfire (I tried swapping buttons, same result)
   The buttons works though and the icon was showing yesterday (before the recent update)

Strange, I didn't change anything that should effect that. Lemme check it out

use these instead:
/cast [@Focus] Sunfire(solar)
/cast [@Focus] Moonfire(lunar)

Your solution works perfectly fine for all circumstances i tested it.
One thing though I just noticed during testing of issue #102 that might help understanding where the "error" comes from and why I didn't notice it earlier:
The macros I used above in the original post work fine (initially and after relogs) until i change speccs and come back into balance. Then the icon is gone, but yet again this is only the case for the sunfire.
Moonfire works perfectly fine even without the "(lunar)".