TL;DR: Drill yourself to move the stick before you push the button.
I have the opposite problem. There's been several times I've tried to Side B and misinputted an Up B. Sucks when you're playing K. Rool. But that's unrelated.
You're probably pressing the button too quickly. Since you're offstage you're probably DI'ing sideways. If you try to press the button and move the stick at the same time, there's a good chance you'll press the button while the stick is still registering as "sideways". Move the stick first, then push the button. Once the move begins you can go back to DI'ing it sideways, just make sure you push the button after the stick is in place.
My sister had a hard time using moves in Pokken, until I drilled her to not try to press the buttons simultaneously, but to push the direction first, then the attack. I'm having to try to teach her the same thing in this game too, because whenever I knock her off to the side she'll side B about 3 times before she inputs an Up B. The only time she's able to recover competently is if she gets knocked directly below the stage, so she didn't have to try to Up B while DI'ing sideways.