In عَلَيْهِمْ مُؤْصَدَةٌ, the first word ends in mīm sākinah and the next begins with a vowelled mīm. This gives idghām shafawī with ghunnah.
Contrast هُمْ يُرَاءُونَ from al-Māʿūn: mīm before yāʾ remains clear. The difference is entirely due to the next letter. If you pause between the words, their cross-word assimilation does not take place.
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ʿAlayhim muʾṣadah: mīm assimilation
Al-Māʿūn and Al-Humazah: Applied Assimilation, Concealment, Madd and Hāʾ Rules
In عَلَيْهِمْ مُؤْصَدَةٌ, the first word ends in mīm sākinah and the next begins with a vowelled mīm. This gives idghām shafawī with ghunnah.
Contrast هُمْ يُرَاءُونَ from al-Māʿūn: mīm before yāʾ remains clear. The difference is entirely due to the next letter. If you pause between the words, their cross-word assimilation does not take place.