Clean SMW 1.26+ installation.
Just realized that the extension works properly when on new pages but seems it doesnt handle the onchange event when editing an existing page with form.
For those who are facing the same issue, as workaround you can replace within src/SemanticFormsSelect.php:
Output::addModule( 'ext.sf_select.scriptselect' );
with
// fellam: workaround handling not loaded module in edit with form
global $wgOut;
$wgOut->addModules( 'ext.sf_select.scriptselect' );
//Output::addModule( 'ext.sf_select.scriptselect' );
Further details hereː https://github.com/SemanticMediaWiki/SemanticFormsSelect/issues/7