When included in an instance, the first token field does not show because Select2 automatically sets the width of the container to 1px. This is a known issue (select2/select2#4176).
In FlexForm, this may have been previously solved in select2.css where .select2-container is explicitly given a width:100% !important. But select2.css sits there unused (ext.wsForm.styles relies on it but ext.wsForm.styles is not called anywhere). It is select2.min.css that is directly called instead but it does not contain the CSS fix.