Select From List and Unselect From List keywords try to select/unselect items both by values and labels. This makes their implementation complex and slow, and the code also seems to have some subtle bugs.
In addition to these keywords, we have dedicated keywords (Un)select From List By Label, (Un)select From List By Value and (Un)select From List By Index which are much more simple. We've decided to deprecate Select From List and Unselect From List keywords in favor of these label/value/index specific keywords.