Some words originally end in sukun but receive a temporary vowel when joined to the next word to avoid a difficult meeting of two still consonants. Such a vowel is a temporary, or incidental, vowel. When the reader stops, the reason for that added vowel disappears and the consonant returns to its underlying stillness.
In «وَلَقَدِ اسْتُهْزِئَ», the dal may be heard with kasrah in connected reading, although its underlying ending is still. A stop on «وَلَقَدْ» restores the sukun; no rawm is applied to that temporary kasrah.
A temporary vowel likewise cannot be treated as an original dammah merely to justify ishmam. The decision depends on the word's underlying ending, not just what is heard in one connected context. This protects rawm from being extended to unsuitable cases.