Rawm and ishmam are both special ways to end a word, but their effects on incidental madd are not the same. In rawm, a faint part of the final vowel is still sounded, so the following consonant is not completely still. In ishmam, the audible recitation has already ended on sukun; the lip gesture follows silently.
With rawm: the extra duration conditioned on a fully still ending is not retained as it would be with full iskan.
With ishmam: the phonetic stop remains complete, so the condition for incidental madd may still apply, provided ishmam itself is permitted.
If the last vowel is not originally dammah, that does not become a valid example of ishmam. Eligibility for the stopping gesture and continued presence of the cause for madd are separate questions. Both are clarified through careful oral practice.