Iqlab and ikhfa shafawi both involve nasal sound before ba, so they can sound similar. But the original letter is different. In iqlab, the source is still nun or tanwin, whose n-like sound is converted toward mim. In labial concealment, the source is already still mim, concealed before ba.
«مِنْ بَعْدِ» — nun + ba: iqlab.
«هُمْ بِهِ» — mim + ba: ikhfa shafawi.
Classifying both merely by looking at the following ba hides the reason for the distinction. First identify the original unvowelled sound, then the following consonant, and apply the appropriate rule.