While we may never know exactly how much of Justice League is Zack Snyder’s and how much is Joss Whedon’s, one thing we know for sure is that the version of Cyborg’s origin in that movie directly contradicts the one shown in Batman v. Superman: Dawn of Justice.
Two lines of dialogue are the problem here. Cyborg discusses the Mother Box — the alien device that both saved his life and gave him his powers — and describes how his father came into possession of it in the first place:
“My father called it the change engine. It was found by the British during World War I. They studied it… but they couldn’t even date it. It’s so old. It was shelved, until the night Superman died. Then she lit up like Christmas. They brought it to STAR Labs…”
The first part of this fits neatly with the previously established history of the Mother Box. The blu-ray release of Wonder Woman featured an epilogue titled “Etta’s Mission.” In this story, Etta Candy recruits Diana’s old allies to recover a “very old and very powerful” artifact and deliver it to the Americans at the end of World War I. This artifact is none other than, yes, the Mother Box.
So far, so good.
The problem arises retroactively in Batman V Superman. During that film’s sequence of Diana watching computer footage of future League members, she sees Cyborg’s transformation via Mother Box. But that footage takes place before Superman’s death.
Let’s go back to the dialogue in Justice League. The Mother Box “lit up like Christmas” on “the night Superman died.” From there, the box is brought to STAR Labs, where Victor Stone’s father uses it to heal and transform his son’s broken body. With that setup, Superman’s death is the inadvertent trigger not only for the creation of Cyborg but by extension the whole JL movie. Unfortunately, the previous film had already established a different timeline; it’s doubtful this will or can ever be reconciled, so the mysterious timing of Cyborg’s origin will have to remain one more head-scratcher from the DCEU.