Neither, mono’s two distracting moves stacked.
Yeah, it’s pretty freaky the first time you encounter that in the arena, but it’s definitely no bug.
When an opponent (Monolophosaurus) uses Distracting Rampage, your attack decreases 50%.
Then, if in the next turn Monolophosaurus uses Distracting Strike (or vice versa), your half-as-powerful attack is reduced to zero. The best counters include creatures with immunity or cleansing moves.
Not a bug, just math.