«مَا جَاءَ أَحَدٌ إِلَّا زَيْدٌ»: complete negative; aḥadun is mentioned, so Zaydun is badal.
«مَا جَاءَ إِلَّا زَيْدٌ»: mufarragh; no exception base appears, so Zaydun is the subject.
Both show ḍammah, but their grammatical functions differ.
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Complete versus Mufarragh
Al-Mustathnā: Connected and Disconnected Exceptions, Four Case Rules
«مَا جَاءَ أَحَدٌ إِلَّا زَيْدٌ»: complete negative; aḥadun is mentioned, so Zaydun is badal.
«مَا جَاءَ إِلَّا زَيْدٌ»: mufarragh; no exception base appears, so Zaydun is the subject.
Both show ḍammah, but their grammatical functions differ.