Super Smash Bros. Ultimate’s latest DLC character, Joker from Persona 5, is freezing player’s Nintendo Switches when they use his Final Smash to end a Classic Mode match.
This issue was confirmed by IGN’s own Zachary Ryan, as his Switch froze three times when finishing a Classic Mode match with Joker’s Final Smash. There are also many other examples, including a Reddit thread posted by CubingVainGlory, that show the freeze happening.
It appears this glitch is only occurring in Classic Mode and only with Joker, and does not appear to impact any other aspect of Super Smash Bros. Ultimate as of this writing.