One possibility is a custom "default setting" was saved for the MediaIn node in the Fusion page. This would be a preference file stored on disk named "MediaIn_Media In.setting" that would have been saved into the folder:
Windows:
%appdata%\Blackmagic Design\DaVinci Resolve\Fusion\Defaults\
macOS:
$HOME/Library/Application Support/Blackmagic Design/DaVinci Resolve/Fusion/Defaults/
Linux:
$HOME/.fusion/BlackmagicDesign/DaVinci Resolve/Fusion/Defaults/
To clear this setting out, you could either:
Option 1) Delete the "MediaIn_Media In.setting" file from your system manually using the Windows Explorer/macOS Finder.
Option 2) You could right-click on a MediaIn node in the Nodes view. In the contextual menu select the "Settings > Reset Default" menu item. This will clear out the customized settings for the MediaIn node.