When any letter other than mim or ba follows a still mim, the result is izhar shafawi: mim must be pronounced clearly at its own articulation point. It is neither assimilated into the following sound nor deliberately concealed. Fluent joining should still preserve each consonant's identity.
«أَنْعَمْتَ»
You bestowed favor. Ta follows mim, so mim is clear.
«عَلَيْهِمْ وَلَا»
Upon them, nor. Waw after mim also requires clear articulation.
Be particularly careful before fa (ف) and waw (و), which use parts of the lips and may tempt an indistinct mim. Articulatory proximity alone does not create concealment.