Hi @Anonymous ,
Could you please share a screenshot about your app's configuration?
Based on the issue that you mentioned, I have made a test on my side, and don't have the issue that you mentioned. Add a Label control within your app, set the Text property to following:
Sum(DataCardValue2.Text, DataCardValue4.Text)
Also please consider take a try to turn off the "Improved app rendering (final validation)" option within Advanced settings of App settings of your app, then re-load your app, check if the issue is solved.
In addition, please also check if the version of PowerApps Mobile App in your Mobile device is the latest version, if not, please consider upgrade it to the latest version firstly, then try it again, check if the issue is solved.
Bets regards,