Grace is unmerited favor. To say that God withholding something unmerited is unjust is to say that God is withholding something He owes. He does not owe anyone His Grace.
I live in Florida so Jeb Bush is my Governor. Let's say there are three inmates due to be executed at midnight. The only power left to stop the executions is a phone call from Gov. Bush giving the condemned prisoner(s) amnesty.
11:50pm rolls around and there hasn't been a phone call. 11:55pm the phone rings and Gov. Bush gives prisoner #1 amnesty. By midnight there has been no other calls and prisoners #2 and #3 are put to death. Was Governor Bush unjust in letting prisoners #2 and #3 die in the execution chamber?
Of course he wasn't. In fact we would think he was most gracious by giving amnesty to the one. Why don't we understand God's grace in the same way? We are condemned prisoners of sin awaiting execution. Some get the amnesty and some don't. That doesn't make God unjust. In fact, that anyone of us gets His Grace is a testimony to His goodness.