You might expect new FilePath(null, "/local/path").getChannel() to be null.

Confusingly, it is instead MasterComputer.localChannel!
If that is fed back into FilePath.<init>(VirtualChannel,String) rather than using .<init>(FilePath,String)
you get an object that behaves strangely and throws exceptions.

core/src/main/java/hudson/FilePath.java

@@ -196,7 +196,7 @@ public final class FilePath implements Serializable {
      *      that's connected to that machine. If null, that means the local file path.
      */
     public FilePath(VirtualChannel channel, String remote) {
-        this.channel = channel;
+        this.channel = channel == Jenkins.MasterComputer.localChannel ? null : channel;
         this.remote = normalize(remote);
     }