Why does the polygon symbology change to hatched lines when the number of classes is changed?

Answer

In ArcGIS Pro 2.2 and later, graduated colors and unique values symbology can be applied to both the outline (or stroke symbol) and fill symbol of a polygon feature layer. This is referred to as the color scheme target, and can be accessed from the Symbology pane by clicking the gear icon next to the color scheme drop-down menu.

In the Classes, Histogram, or Scales sub-tabs of the Symbology pane, if the user changes the number of classes, clicks Refresh values, or Regenerate all symbols, the polygon symbol fills change to a hatched pattern. Diagonal lines draw inside each polygon in the feature layer.

This is expected behavior. Although applying the same color to both a polygon's outline and fill can be cartographically pleasing, it functionally serves to allow updating all symbols at once. For example, applying the color scheme to only the outline allows the user to customize the fill of each symbol.

Edits made to the data, such as newly added polygons, do not have the given symbology applied to them until the symbols are regenerated. When the symbols are regenerated (changing the number of classes requires regenerating all symbols), any solid-fill polygon that overlaps or exists beneath other existing polygons could theoretically become invisible. Thus, a hatch fill is applied to all polygons to show these polygon features and potentially resolve data mapping issues. This is especially common in wildfire activity maps.

To return to regular polygon fills, click the Color scheme options button (gear icon), change the option back to Apply to fill, and click More > Regenerate all symbols.