Praise be to Allah, and may peace and blessings be upon the Messenger of Allah.
There is no contradiction in the verse. The statement of the people of Hell:
“whom we used to count among the wicked”
means that they regarded them as such in this worldly life. In other words, they mistakenly believed that the people of faith were evil and mocked them.
Their statement:
“Did we take them in mockery?”
is an acknowledgment and expression of regret for what they had done in this world. It has also been said that the question here is one of reproach and astonishment, as though they were saying: How could we have mocked them and judged them wrongly?
As for the claim that when a disbeliever witnesses the punishment of the grave, he thereby learns the fate of the believers, there is no evidence that the inhabitants of the graves know the detailed destinies of others. Rather, the deceased is questioned about matters concerning himself, and his situation involves what he experiences of blessing or punishment.
Even assuming that some of the inhabitants of the graves might learn something about the fate of another person, it does not necessarily follow that such knowledge would remain present when the Hour is established. The terrors of the Day of Resurrection are immense. Allah تعالى says:
“O mankind, fear your Lord. Indeed, the convulsion of the Hour is a tremendous thing. On the Day you see it, every nursing mother will be distracted from the child she was nursing, every pregnant woman will deliver her burden, and you will see people as though they are intoxicated, although they are not intoxicated; but the punishment of Allah is severe.” [Al-Hajj: 1–2]
One of the strongest interpretations of the verse is also that the question of the people of Hell is not merely a request to know where the believers are. Rather, it is a question arising from regret, sorrow, and self-reproach. They used to mock the believers in this world, then they found themselves in Hell while seeing that those whom they had mocked had been saved.
It is as though they are saying: Where are those whom we used to regard as wicked? Were we wrong when we mocked them, or are we simply unable to see them? All of this is an expression of the intensity of their regret and remorse over what they had neglected and done.
In conclusion: The fact that the people of Hell ask about the believers does not necessarily mean that they did not know the truth in the grave. Nor does it necessarily mean that they had already learned the fate of the believers there. The verse is understood as expressing their regret and sorrow over their attitude toward the believers in this worldly life, and there is no contradiction between it and the conditions of the grave or the terrors of the Day of Resurrection.
And Allah تعالى knows best.