After recently updating from an outdated version of QGIS to the latest Long Term Release, 3.16.13, the behavior of Batch Processing for the Refactor Fields vector tool has changed. Each time a layer is added, the prompt above appears asking, "Do you want to reset the field mapping?" The answer in my case is always "Yes," since I would be using "Calculate field expression" to populate the Fields mapping column afterwards. However, in some cases, I may be using "Add layers by expression" to add hundreds of vector layers to the Input layer column, and pressing "Yes" hundreds of times even using "Y" as a keyboard shortcut is cumbersome. Is there any way to turn off these "Are you sure?" type prompts for Batch Processing functions, or set a default behavior?
AFAIK, it's not possible.
You could file an issue report or a feature request in the QGIS repository on GitHub https://github.com/qgis/QGIS/issues.