I ran into a situation within ArcGIS Pro where I was trying to select by polygon. When you use either select by polygon, line, or trace... all functions that require multiple user inputs to finish the shape... and you try to move the map prior to finishing the selection it clears the polygon, line, or trace feature that you started.
ArcMap doesn't function this way, Web Applications does not work this way.... even the ArcGIS Pro Measure tool does not work this way.
The workaround is to finish the selection. Move the map. Hold the shift or ctrl button. Select another section. Repeat all those steps.
ESRI is making the terminology similar between ArcGIS Pro and Web Apps ..... like Popups... and the integration with ArcGIS Online. My suggesstion would be to make the ArcGIS Pro Selection functionality just like the measure functionality.
This is a Duplicate of ArcGIS Pro: Allow panning and zooming while using the "Select by Polygon" tool and is fixed in ArcGIS Pro 2.3 slated for release in January 2019.