I have copied my TrainingArea many times, and while training, it seems to me that when other agents eat their food, it disappears from the first agent. I may be wrong and it may not be. But I know for sure that the food disappears by itself from the very first agent.

here's my TrainingArea.cs:
using UnityEngine;
using System.Collections.Generic;

public class TrainingArea : MonoBehaviour
{
    [SerializeField] private GameObject playerPrefab;
    [SerializeField] private GameObject foodPrefab;
    [SerializeField] private int numberOfFoods = 10;
    [SerializeField] private float spawnRadius = 5.0f;
    private PlayerAgent playerAgent;
    private List<GameObject> foodInstances = new List<GameObject>();

    private void Start()
    {
        SpawnPlayer();
        SpawnFoods();
    }

    public void ResetArea()
    {
        ResetPlayer();
        ResetFoods();
    }

    private void ResetPlayer()
    {
        if (playerAgent != null)
        {
            playerAgent.transform.localPosition = new Vector3(Random.Range(-spawnRadius, spawnRadius), Random.Range(-spawnRadius, spawnRadius), 0);
        }
    }

    public void ResetFoods()
    {
        List<GameObject> eatenFoods = new List<GameObject>();

        foreach (GameObject food in foodInstances)
        {
           if (food != null)
           {
               food.transform.localPosition = GetValidSpawnPosition();
           }
           else
           {
               eatenFoods.Add(food);
           }
        }

        foreach (GameObject food in eatenFoods)
        {
            Vector3 spawnPosition = GetValidSpawnPosition();
            GameObject foodInstance = Instantiate(foodPrefab, spawnPosition, Quaternion.identity, transform);
            foodInstances.Add(foodInstance);
        }

        // Remove eaten food references from the list
        foodInstances.RemoveAll(food => food == null);
    }

    private void SpawnPlayer()
    {
        Vector3 randomPosition = GetValidSpawnPosition();
        GameObject playerInstance = Instantiate(playerPrefab, randomPosition + transform.position, Quaternion.identity, transform);
        playerAgent = playerInstance.GetComponent<PlayerAgent>();
    }

    private void SpawnFoods()
    {
        for (int i = 0; i < numberOfFoods; i++)
        {
            Vector3 spawnPosition = GetValidSpawnPosition();
            GameObject foodInstance = Instantiate(foodPrefab, spawnPosition, Quaternion.identity, transform);
            foodInstances.Add(foodInstance);
        }
    }

    private Vector3 GetValidSpawnPosition()
    {
        Vector3 spawnPosition;
        do
        {
            spawnPosition = new Vector3(Random.Range(-spawnRadius, spawnRadius), Random.Range(-spawnRadius, spawnRadius), 0);
        } while (playerAgent != null && Vector3.Distance(playerAgent.transform.localPosition, spawnPosition) <= 5.0f);

        return spawnPosition;
    }

    public bool AllFoodEaten()
    {
        foreach (GameObject food in foodInstances)
        {
            if (food != null)
            {
                return false;
            }
        }
        return true;
    }
}


here's my PlayerAgent.cs:
using UnityEngine;
using Unity.MLAgents;
using Unity.MLAgents.Actuators;
using Unity.MLAgents.Sensors;

public class PlayerAgent : Agent
{
    [SerializeField] private float moveSpeed = 5.0f;
    [SerializeField] private float maxEpisodeTime = 20.0f;
    private TrainingArea trainingArea;
    private float episodeTime = 0.0f;

    public override void Initialize()
    {
        trainingArea = GetComponentInParent<TrainingArea>();
    }

    public override void OnEpisodeBegin()
    {
        trainingArea.ResetArea();
        episodeTime = 0.0f;
    }

    public override void CollectObservations(VectorSensor sensor)
    {
        sensor.AddObservation(transform.localPosition);
    }

    public override void OnActionReceived(ActionBuffers actions)
    {
        float moveX = Mathf.Clamp(actions.ContinuousActions[0], -1, 1);
        float moveY = Mathf.Clamp(actions.ContinuousActions[1], -1, 1);

        Vector3 move = new Vector3(moveX, moveY, 0) * moveSpeed * Time.fixedDeltaTime;
        Vector3 newPosition = transform.localPosition + move;

        Collider2D collider = Physics2D.OverlapCircle(newPosition, 0.5f, LayerMask.GetMask("Wall"));
        if (collider == null || !collider.CompareTag("Wall"))
        {
            SetReward(-0.1f);
            transform.localPosition = newPosition;
        }

        Collider2D collider2 = Physics2D.OverlapCircle(newPosition, 0.5f, LayerMask.GetMask("Food"));
        if (collider2 != null && collider2.CompareTag("Food"))
        {
            Debug.Log("OnTriggerEnter2D");
            SetReward(1f);
            Destroy(collider2.gameObject);
        }
    }

    public override void Heuristic(in ActionBuffers actionsOut)
    {
        ActionSegment<float> continuousActions = actionsOut.ContinuousActions;
        continuousActions[0] = Input.GetAxis("Horizontal");
        continuousActions[1] = Input.GetAxis("Vertical");
    }

    private void FixedUpdate()
    {
        episodeTime += Time.fixedDeltaTime;
        
        if (episodeTime >= maxEpisodeTime || trainingArea.AllFoodEaten())
        {
            EndEpisode();
        }
    }

}