Not a bug, here's the list of things that add +1 mistake.
1. Hunger ≥ 1
2. Health ≤ 25%
3. Sleepiness ≥ 3
4. Happiness ≤ -1
5. Discipline ≤ -1
6. Bond ≤ 0
7. Poops in Device ≥ 1
8. Weight ≥ 200%
The starting Zurumon that comes with the game starts with Bond=0, so that's why you'll see that Mistake=1 until the Bonding Mechanics are implemented.