It's no secret that one of the upcoming Marvel Cinematic Universe movies is undergoing reshoots despite only being a few months from release. Doctor Strange in the Multiverse of Madness has been getting a slight update as of late, as has been widely made clear, even by lead actor Benedict Cumberbatch during his appearances to hype up the release of No Way Home, but as for the reason for these reshoots, it turns out the multiverse itself is to blame.
As mentioned in a The Hollywood Reporter newsletter, it was stated that Marvel was "jazzed" by the many crossovers in No Way Home and in Loki, and therefore want to have "more fun with the multiverse" than they originally did the first time around. It's also noted that this will include "more cameos and character introductions", but as for who they are, as was the case with No Way Home, we'll just have to wait to see for ourselves.
As for when Doctor Strange in the Multiverse of Madness will open, the movie is still slated for its delayed release date of May 6, 2022.