When a Muslim commits kufr their deeds do not become null and void. Committing an act of kufr doesn't make one a kafir meaning a disbeliever. Such people are sinful but not disbelievers.
Incase someone apostates entirely from Islam their deeds become null and void. Although, if they come back to Islam their deeds will be restored and this is from the immense mercy of Allah Ar Rahman Ar Raheem.
Shaykh Navaid Aziz answers and explains...
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