Ok, so this is the first time I came across the mover "Powder" in a Pokémon battle. I was on Pokémon Showdown! when my opponent's Vivillion used it on my Volcarona and the "powder exploded". However, before I KOed it with a Bug Buzz, it another Powder off, and the status remained on my Volcarona throughout the battle and it didn't explode when I used a Fire move after Vivillion fainted. Do the effects of Powder still work even when the opponents switched out, or is it a glitch?
Here's the replay:
What you witnessed was a graphical error on Showdown's side, meaning that it still showed that you had "Powder on you" when you didn't.
Powder only works on the turn it was used.
It should only work on the same turn as you used it. If they use a fire type attack that turn it will activate, otherwise it won't.
So this was not a glitch, it's just normal.
Hope I helped.