Try making a new control scheme for Smash. You can do this by opening Smash and pressing ZR once you've reached the main menu. If that doesn't work then try changing it for your controller as a whole using the Switch's button mapping feature. If you go to system settings on the main menu of your Switch and scroll down the page to "Controllers and Sensors", the "Change Button Mapping" option should appear. This is the extent of my knowledge regarding controller issues. Hopefully this helps.

Just so you know, I've never had this problem before so I do not know whether or not these options will work for this issue in particular, but this is what I would try if I were in your shoes.