The rotation property for labels in a mobile map package (.mmpk) created in ArcGIS Pro 2.3 is not respected in the latest version of AppStudio for ArcGIS.
This issue does not exist with AppStudio 3.3 as it uses ArcGIS Runtime 100.5, on which the issue is not reproducible. The best course of action to resolve the issue would be to upgrade AppStudio to version 3.3.