Zack Snyder's Justice League had Martian Manhunter make a big cameo that has people questioning where he's been, but it doesn't actually cause a DCEU plot hole as some might think. One of the biggest differences with the Snyder Cut from Justice League's theatrical version was the inclusion of Harry Lennix who has been part of the DCEU since 2013. While Lennix played General Swanwick in Man of Steel and Batman v Superman: Dawn of Justice, his role within this version of the DC Universe became something far bigger. Despite it not being the original plan, Snyder revealed in the Snyder Cut that Swanwick was none other than J'onn J'onzz a.k.a. the Martian Manhunter, bringing another big Justice League hero into his universe.
Martian Manhunter's screen time in Zack Snyder's Justice League may have been brief, but all of his scenes were imperative nonetheless. While Lois Lane is still grieving the death of Superman, J'onn disguises himself as Martha Kent, one of the only people who could get through to Lois in that moment, to try and help her get back in the world. As Martian Manhunter's words eventually do help Lois start to try to go back to her life, the Snyder Cut never addresses J'onn having posed as Clark Kent's mother. J'onn's only other scene is at the film's final moment where he goes to see Batman and tells him the Justice League inspired the martian to come out of the shadows and start contributing to protecting Earth as Darkseid was still coming.
Does this mean Zack Snyder's Justice League left a big plot hole when it comes to the real Martha? One of the big aspects of Martian Manhunter is his telepathic powers which get important in regards to his arc in the Snyder Cut. When J'onn (as Martha) sees Lois, he's telling her about how Ma Kent lost the farm and moved to a small place by the diner. The way J'onn would have found this out is by having read Martha's mind at some point prior to his conversation with Lois.
That's also how J'onn would have found out that Martha had gone to the Daily Planet to pick up Clark's belongings while also finding out that Lois hadn't been to work since he died. But while Lois and the real Martha do eventually meet up later in the film, the conversation is never brought up. While it can certainly look like Zack Snyder's Justice League left a big plot hole with no conclusion, it wouldn't be difficult to solve it. All Martian Manhunter would have to do is plant memories with Martha of a conversation with Lois. Then, it would be as if the real Martha talked with Lois about finding the light again in her life after losing Clark.
Because even if Zack Snyder's Justice League never deals with it, it'd be out of character for Martian Manhunter to forget that this conversation would eventually come up between the real Martha and Lois would have met up. It did also make more sense for J'onn to pose as Martha rather than come to Lois as Swanwick. Despite Swanwick and Lois having a working dynamic from past films, she would likely not have opened up to him the way she did to Martha because it's a different relationship. But despite it looking like they left that plot unfinished, Martian Manhunter likely did make sure to take care of it offscreen in Zack Snyder's Justice League.
