I've spent some quality time with Pro identifying its deficiencies in the context of my personal workflows and ran across this. I took a layer of cities and tried to label the polygons using the name field which has the city name but stored as all caps ("LOS ANGELES", "SAN DIEGO", etc).
No matter- I'll just add an Arcade expression and convert the labels to proper case on the fly:
Hmm. My labels are still all caps. This goes on for awhile. I tried different permutations of label expressions, VBScript vs Arcade, etc and I still could not get anything but upper case labels. I finally noticed that the Text Symbol Style I had selected was the Landform/Physical Region style, which shows an example of two letter As, both upper case. I changed my text symbol style to Landmark/POI and I finally get proper case labels.
So- was this by design or a bug? I didn't see anything in the help documentation that Arcade (or any label expression) will not override properties of the selected Text Symbol Style. I'm using Pro v2.1.0