I think I know why this is happening. I'm not entirely sure, but I believe that's the reason.

It seems the durations array contains objects with properties like days and lemonId. These should ideally be numbers, but if they're not being parsed correctly on the frontend, they might end up as strings with invalid characters—such as single quotes or other non-numeric elements—before being sent.

Alternatively, it could be a network issue. Even if the frontend is sending perfect JSON, network glitches might corrupt it. I know this because it happened to me.

If neither of those is the issue, it might be due to middleware or proxies between the frontend and the API route. Another possibility is that the frontend is sending data with non-ASCII characters.