Since version 0.9, the Sender header is displayed in the message view as well as the From header was before. If both are set, I can't distinguish them because both carry the label "Sender" in the UI. Not sure how to fix that. "From" and "Sender" have different meanings, labelling one of them as the other is probably not a good idea.
are you sure the labels file for your selected localisation is up to date? if you look in en_US/labels.inc you should see separate labels for from and sender.
Am I sure enough to install Roundcube in an empty directory from the original download archive? My language is de-DE.