Compare مَسَاجِدُ with كُتُبٌ. Both are broken plurals, but مَسَاجِدُ has the required صيغة منتهى الجموع form and is ghayr munsarif. كُتُبٌ does not have that form, so it remains declinable and takes tanwin.
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A Broken Plural Is Not Automatically Diptote
Cause of Ghayr Munsarif: Sighat Muntaha al-Jumu'
Compare مَسَاجِدُ with كُتُبٌ. Both are broken plurals, but مَسَاجِدُ has the required صيغة منتهى الجموع form and is ghayr munsarif. كُتُبٌ does not have that form, so it remains declinable and takes tanwin.