After many minutes trying to understand why the title case label expression we have built and use in other projects in both map and pro to modify our all caps street names was validating but not changing the labels, we discovered the setting under Label Class > Symbol > Appearance was set to "Upper case". On changing the setting to "Normal", the expression worked. So.... the appearance setting overrides the expression. Is this expected behaviour or a bug? ArcGIS Pro 2.1.2