Creating a thumbnail does not create a new thumbnail in the ArcGIS Pro metadata.

Additional Information

Workaround

Capture the map's thumbnail using this workaround. In the map view's Contents pane, right-click the map and click Properties in the context menu. In the Map Properties dialog box, click Metadata on the left, and at the bottom of the dialog box on the right, click the Generate Thumbnail button.

Steps to Reproduce