In /styles/prosilver/template/posting_editor.html line 32 reads:

<!-- IF bcc_recipient.IS_GROUP --><a href="{bcc_recipient.U_VIEW}"><strong>{bcc_recipient.NAME}</strong></a><!-- ELSE -->{bcc_recipient.NAME_FULL}&nbsp;<!-- ENDIF -->

...in contrast to line 20 which reads:

<!-- IF to_recipient.IS_GROUP --><a href="{to_recipient.U_VIEW}"><strong>{to_recipient.NAME}</strong></a>&nbsp;<!-- ELSE -->{to_recipient.NAME_FULL}&nbsp;<!-- ENDIF -->

The former (line 32) misses a nbsp in front of the <!-- ELSE -->.

Interestingly enough I fail to find the ability to add a group as BCC recipient in a vanilla installation. Discovered this by chance when modifying both the template and one of the UCP files to add a checkbox named "Add [BCC]" near the group box (which does what its named like).