So I'm doing the Noel gym fight, and have got to his last pokemon, the Clefable. The weather is raining, and the field is grassy. At the start of the battle I set up toxic spikes twice so the Clefable is badly poisoned. However, at some point (I didn't catch when it happened) the Clefable stopped taking damage from poison at the end of the turn. There is the message about rain, and the message about pokemon being healed by the field, and the message about Clefable healing from the ingrain affect of it's held seed, but no message (and no health bar movement) related to the Clefable taking poison damage.
Not game-breaking, and my party is in a position to finish the battle without the Clefable taking poison damage, but could understandably ruin other player's days if the poison status is broken.
Please let me know if you would like more information about my game when the bug occurred
N.B. In going back to finish the fight, the Clefable returned to full health meaning that the two healing effect messages didn't appear. The Clefable still did not take damage even though the other messages were not displayed.