In مَضْرُوبٌ, the root is ḍ-r-b, not m-ḍ-r. The mīm and wāw belong to the مَفْعُولٌ pattern, while the root fills the f-ʿ-l positions.
Word / formMeaning / explanation
مَنْصُورٌroot n-ṣ-r.
مَسْمُوعٌroot s-m-ʿ; the second mīm is an actual radical.
مَكْتُوبٌroot k-t-b.
A visible mīm is not automatically extraneous: compare the word with its underlying root and pattern.