Unchecking Show Label not Working. This is from a Menu button added to an existing toolbar. By toggling the toolbar label state to off and then back to default the label stayed off.
Go to Settings > Customize Toolbars > Toolbar and select the toolbar that button is on.
Make sure Label State = Default. If it is On or Off then it will override the options within individual buttons.
It had been default--double-checked before I toggled it but can't replicate the behavior now.