What build of Rockbox are you using? A daily/revision build, a release (like 3.6, 3.7, 3.7.1), or a custom build (compiled by yourself)?
If it is a build taken from "current build" section, try to update to the latest one and see if it still happens.
IMO this is a bug, either in the theme (that isn't defining the tag properly), on the theme parser, or on the code that passes to the theme the currently playing time.