I believe this was confirmed as Canon. In so much that Steven does finally get a hold of controlling his shield. It was also stated that this happens before the whole Jailbreak/Return episode so at least it doesn't mess up too much with what they got going.Scarim Coral said:For those who has doubt about the upcoming crossover episode, here is a preview of it-
I myself never seen Uncle Grandpa nor do I really want to watched it. Also from what I read, it was Rebecca herself who approach the crossover idea and it is not canon to Steven Universe world but don't quot me on this as I don't have a source on it (read it from a tumblr post).