Joss Whedon committed the same mistakes from Justice League. The filmmaker took over the project from original helmer Zack Snyder after he had to step away from the Warner Bros. project due to a personal tragedy. But it became immediately clear that this wasn't the right decision.
Justice League's tumultuous production has been widely publicized since its release. Due to the divisive response to drastically different final cut.
This was immediately apparent resulting in demands to roll out Snyder's original version of the movie — even if it wasn't fully complete. As it turns out, fans are the only ones who see Whedon's edit of Justice League as non-canon, other DCEU directors also discard it from continuity. Wonder Woman 1984 helmer Patty Jenkins candidly revealed this recently, adding that she doesn't consider it canon as it contradicts what was established in Diana Prince's 2017 origin story and its sequel, essentially creating a continuity lapse in the DCEU. Somehow, this was also what happened with the MCU through Age of Ultron.
Marvel Studios hired Whedon for their ensemble film, Avengers: Endgame with Smart Hulk going through a mild meltdown upon learning Natasha Romanoff's death for the Soul Stone in Vormir, but other than that, the specific plot point never really factored in the pair's respective arcs following Ultron.
Similarly, DCEU filmmakers who picked up their individual heroes for their own solo movies had to either totally ignore what Whedon's change did to Justice League's characters, or find a way to explain it. Given this additional challenge brought by Whedon veering away from Snyder's plan which was already part of the DCEU's blueprint as other filmmakers worked on their standalone offerings, it makes sense that they're not particularly thrilled with the 2017 film.
While the Snyder Cut will finally release on HBO Max, Whedon's Justice League is still considered canon in the DCEU for now, but unlike the MCU and Avengers: Age of Ultron, the DCEU's multiverse approach could change all that, especially if the Snyder Cut is a massive hit.
