That's actually quite a believable way for it to happen: Normally realising Yuuki is dead is something she doesn't want to acknowledge and thus will fight against it, like happened in the hospital. However, Yuuki is a projection of a sort and while it's not the real Yuuki anymore, it still should have a degree of increased freedom compared to Mirai's normal thought processes. So, the imagined Yuuki could say things Mirai doesn't want to hear but still knows to be true. Kind of like externalising it so that she doesn't need to say it first herself.