Single term option is ignored about line 1354 in SmartyTermList():

Original:
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if( $this->GetPreference('disable_prettyurls',0) == 0 )
 {
  $prettyurl = 'glossary/'.$row['term_id'].'/'.$returnid;
}

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The preference 'show_single' is not checked. However show_single should form
prettyurl with glossarysing, so that custom template is taken.