Following his stint as Quicksilver in the X-Men series, Evan Peters got to make his Marvel Cinematic Universe debut in WandaVision. The actor played a new version of Pietro, who was later revealed to be an (unwitting) impostor named Ralph Bohner. Prior to the twist being unveiled, many fans were surprised by what appeared to be a 20th Century Fox/X-Men character appearing in a Marvel Studios project.
As it turns out, one of those surprised by Peters' role was longtime X-Men writer, producer and director Simon Kinberg, who entered the X-Men franchise with 2006's The Last Stand, and continued to be involved in the series until its end with New Mutants. Despite his history with the franchise, the director revealed to Screen Rant that he only learned about Evan Peters' WandaVision role at the same time as everyone else:
"[I] did not know it was going to happen in advance, actually, which was wild [...] I didn't even see it in real time. I was late to it by a day or two, but obviously, I have Twitter and I also have two sons who were hardcore X-Men and Marvel fans, and they saw it and they immediately texted me [...] 'Dad, did you see?' And I was like, 'No, I haven't seen the ep. Thanks for spoiling it.'"
As someone who was closely associated with the X-Men movies, Kinberg's apparent surprise at not being clued in to Peters' role in WandaVision is understandable. Having said that, it's not exactly a shock that Marvel Studios didn't give him a heads-up, seeing how WandaVision was entirely separate from the X-Men saga, and the Quicksilver in the show was not the Pietro from Fox's franchise.
Furthermore, Marvel Studios is famously secretive about its projects, so it's not surprising for the news to have been kept for those working on the show in order to keep the surprise for viewers as long as possible — despite the fact that Peters' role leaked around the time that WandaVision came out.
Speaking of Wanda and surprise X-Men crossovers, rumors have been swirling that the Scarlet Witch will fight a pivotal X-Men character in Doctor Strange in the Multiverse of Madness. Even though said rumors should be taken with tiny a grain of salt, they are interesting to ponder while we wait for the MCU to delve into the multiverse with the Doctor Strange sequel.