While in the Hoopa Dungeon, I used Dig on my Drillbur to attack the Tentacruel. While underground, Tentacruel used Toxic and poisoned me.
 

Is this a bug?

 

If a Poison-type Pokémon uses Toxic, any moves it uses for the rest of the turn will be guaranteed to hit, even if a target is in the semi-invulnerable turn of a move (such as Fly or Dig)

Hi. Thank you for contacting us about your issue.

 

Indeed, as @Mysticwind kindly pointed out, Toxic used by a Poison-type Pokémon will guarantee to hit the opposite Pokémon even if they're in a semi-invulnerable state (Fly, Dig, even Phantom Force I believe). If your Pokémon is a Steel or Poison type, it won't get poisoned obviously. This has been the case since Generation IV.

 

We hope the information provided will be useful to you. Do not shy away to come back and ask if you have any future questions. Good luck in your adventures!

 

- Kubrick