The first canto describes how Ashvatthama, having killed the Pandavas’ sons, runs for his life and, when he is caught by Arjuna, then releases the brahmastra. This is a curious incident for a few reasons. Firstly, when Arjuna sees the weapon’s glare he is bewildered. This seems odd, as Arjuna was a highly accomplished warrior who knew and indeed had already used the brahmastra weapon. I admit that my knowledge of astras is scant, and maybe there are different forms of the brahmastra and this was one he had not seen. But it does seem strange, especially as Krishna, having told Arjuna that Ashvatthama had released the weapon , then tells him to fire his own brahmastra and withdraw both.
Anyway, that aside, Arjuna does as Krishna asked. We then hear how Ashvatthama was duly punished and sent away to the forest. But then the Bhagavatam goes on to describe how Uttara ran in panic to Krishna as he was leaving for Dvaraka (it seems), some time later. She said that a “fiery iron arrow” was approaching her and this is identified as the brahmastra of Ashvatthama, who by now should be skulking in the forest somewhere.
So if Arjuna withdrew this weapon how did it carry on toward Uttara? Could Ashvatthama have released another brahmastra? That seems improbable, as he was divested of his power by having his topknot jewel removed (1.7.56), an act intended to render him impotent and thus no longer a threat (“killed and not killed”). Would Krishna and the Pandavas have left such a heinous fellow in a position to do more harm after they had punished him?