In ArcGIS Pro, the Field Map parameter of the Merge geoprocessing tool does not include all fields from the input datasets in the output fields by default.

Workaround

All fields are added immediately when: 

• Using the Add Many tool.
• Drag and dropping data from Contents.
• Browsing to add the inputs.

Alternatively, all fields can be added using the Reset button on the upper right of the Field Map pane after the inputs were added from the input dropdown widget.

Steps to Reproduce