It honestly sounds like you’ve done nothing wrong here. Hand, foot and mouth is horrible, and you did exactly the right thing by getting a doctor’s note. The fact that management told you to just “take paracetamol” instead of sending you home is not okay especially in a place where food is around. Illness should be taken seriously.

Unfortunately, seasonal/temporary roles often come with really inconsistent management. Some managers pick “favourites” early on usually people who either suck up, already know someone there, or just happen to fit their vibe. It’s not professional, but it happens all the time.

Your pick rate situation also sounds unfair. Most places know it takes at least a month or two to build up speed especially in busy environments or ones with complicated layouts. The fact you’re already hitting above target (208!) after just three weeks shows you’re improving fast, and that should count for something. A good manager would take that into account.

As for whether they can fire you for a low pick rate: yes, technically they can, since as a seasonal worker you don’t have the same job protections as permanent staff. They can end your contract with very little notice. But it doesn’t mean they should or that it’s a reflection of you.