I don't understand why a non answer simply asserting that Advaita is correct is accepted as an answer. It seems to be completely irrelavant.

Anyways let me reclarify your question, you are asking since an Omniscient God needs to know everything, and knowing would imply learning which would mean there was a period when God didn't know a particular thing and was ignorant, this also implies there is change happening in God. As ignorance and changes are blemishes in God, God cannot be omniscient.

Response: The above argument is wrong because it assumes there was a period in the past when God did not know something and then he had to learn. This is not the Hindu conception in my opinion. This is just projecting human attributes which are flawed and limited onto the God who is prefect and infinite.

God has always been omniscient, he has been omniscient from eternity, he didn't have to learn anything because he has always known everything.

There is nothing else but thou, O lord; nothing else has been or will be. Thou art both discrete and indiscrete, universal and individual, omniscient, all-seeing, omnipotent, possessed of all wisdom and strength and power.
Vishnu Purana - 5.1.47

Nārāyaṇa is the only one that is stainless, sinless, changeless, and unnameable, and that is pure and divine. There is no second. Whoever knows Him thus, becomes Vishṇu Himself. The Yajurveḍa teaches this. NĀRĀYAṆA-UPANISHAD OF KṚSHṆA-YAJURVEḌA

Never was there a time when I did not exist, nor you, nor all these kings; nor in the future shall any of us cease to be. Bhagavat Gita - 2.12

So if God is always present, eternal and changeless and omniscient it means he was always omniscient.

Question: Since an Omniscient God needs to know everything, and knowing would imply learning which would mean there was a period when God didn't know a particular thing and was ignorant, this also implies there is change happening in God. As ignorance and changes are blemishes in God, God cannot be Omniscient.

Response: The above argument is wrong because it assumes there was a period in the past when God did not know something and then he had to learn. This is not the Hindu conception in my opinion. This is just projecting human attributes which are flawed and limited onto the God who is prefect and infinite.

God has always been omniscient, he has been omniscient from eternity, he didn't have to learn anything because he has always known everything.

There is nothing else but thou, O lord; nothing else has been or will be. Thou art both discrete and indiscrete, universal and individual, omniscient, all-seeing, omnipotent, possessed of all wisdom and strength and power.
Vishnu Purana - 5.1.47

Nārāyaṇa is the only one that is stainless, sinless, changeless, and unnameable, and that is pure and divine. There is no second. Whoever knows Him thus, becomes Vishṇu Himself. The Yajurveḍa teaches this. NĀRĀYAṆA-UPANISHAD OF KṚSHṆA-YAJURVEḌA

Never was there a time when I did not exist, nor you, nor all these kings; nor in the future shall any of us cease to be. Bhagavat Gita - 2.12

So if God is always present, eternal and changeless and omniscient it means he was always omniscient.