Foreign ministry of Pakistan is an authority here over this issue . So , we must go with what they say ,that is , Zafrullah Khan.
If you see wikipedia page of Zafrullah Khan , he is the first foreign minister of pakistan,and Liaquat Ali khan is regarded as First primeminister of Pakistan. Though he had portfolio of foreign ministry under his control, but officially Zafarullah khan is regarded as the first foreign minister of Pakistan ,as per MOFA.
It's a strange thing that you have noticed , because interim government ended on 14 august and Liaquat Ali khan took charge of Prime minister and foreign office on 15th of August , but he is not regarded as first foreign minister,maybe due to his position as a first primeminister,because once Nawaz Sharif also had MOFA under his control,without having any foreign minister ,then sartaj aziz became advisor after some time. Nawaz Sharif's name is not shown as a foreign minister too, at wikipedia and MOFA but sartaj aziz's name is there. So , we come to a conclusion that Liaquat Ali khan was a first prime minister,though he had a direct charge of foreign ministry under him,without having any foreign minister that time,this is why Zafarullah khan is regarded as first foreign minister of Pakistan.

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''Sufficient for us is Allah , and He is the best Disposer of affairs" Al-Quran