مَسَاجِدُ, مَصَابِيحُ, and دَوَابُّ have final ḍamma but no tanwīn, because their special broken-plural pattern causes diptote status.
Compare fully declinable plurals such as كُتُبٌ and رِجَالٌ.
Yet a lack of tanwīn by itself does not prove diptote status; definite and construct nouns can also lack it.
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Diptote Causes — Ultimate Broken-Plural Patterns
مَسَاجِدُ, مَصَابِيحُ, and دَوَابُّ have final ḍamma but no tanwīn, because their special broken-plural pattern causes diptote status.
Compare fully declinable plurals such as كُتُبٌ and رِجَالٌ.
Yet a lack of tanwīn by itself does not prove diptote status; definite and construct nouns can also lack it.