When a madd letter is followed by a consonant that becomes still only because of a pause, madd ‘arid li-s-sukun can arise. But with rawm, a small portion of the final vowel remains audible; the consonant is no longer entirely still in the same sense as a full stop. Consequently, the longer incidental-madd treatment used with full iskan is not applied in the same way.
If «الرَّحِيمِ» is ended with rawm of the kasrah, the full incidental-sukun condition is no longer present.
The same distinction applies when the final dammah of «نَسْتَعِينُ» is given rawm.
Two questions must be linked: how much of the last vowel remains? and what exactly is causing the preceding madd to extend? Under rawm, the basic vowel length is retained but the additional length of fully stopped incidental madd is not used.