Same thing happens to me but only if I use %{site_name} in the translation. The problem seems to be the misleading default translation. If you instead use %{email_prefix} Your translation it will work the way you want it to.
I am looking at the code for the first time, but if this does what I think it does that means SiteSettings.title is provided to the translation as ‘email_prefix’ because of customizable email branding settings.
I’m not really sure where the misleading default comes from because, in the case of my language at least, there is no default provided and the default for english uses email_prefix.