Why does the familiar مَسْجِدٌ have kasrah although يَسْجُدُ has ḍammah?

Ism al-Ẓarf: Meaning, Patterns, Paradigm and Derivation

• It is a comparative pattern
• It is an active-participle plural
• It is an attested lexical form differing from the basic rule
• It is a perfect-tense verb