because you reload the scene, those

[turbid notch]

no

[rustic lynx]

Let me find out

[turbid notch]

learn how to get a reference
or don't make your game manager DDOL and instead just include it in the scenes you need it in

[rustic lynx]

Wait, i found it

using UnityEngine;
using UnityEngine.SceneManagement;
using TMPro;

public class GameManager : MonoBehaviour
{
    public static GameManager instance;

    public GameObject gameOverUI;
    public TextMeshProUGUI restartCountdownText;
    private PlayerShooting playerShot;
    private FPSController fpsController;
    private PauseController pause;

    private float initialCountdownTime = 10f;
    private float currentCountdownTime;

    private void Awake()
    {
        if (instance == null)
        {
            instance = this;
            DontDestroyOnLoad(gameObject);
        }
        else if (instance != this)
        {
            Destroy(gameObject);
        }
    }

    private void OnEnable()
    {
        SceneManager.sceneLoaded += OnSceneLoaded;
    }

    private void OnDisable()
    {
        SceneManager.sceneLoaded -= OnSceneLoaded;
    }

    private void OnSceneLoaded(Scene scene, LoadSceneMode mode)
    {
        playerShot = FindObjectOfType<PlayerShooting>();
        fpsController = FindObjectOfType<FPSController>();
        pause = FindObjectOfType<PauseController>();
    }

    private void Start()
    {
        playerShot = FindObjectOfType<PlayerShooting>();
        fpsController = FindObjectOfType<FPSController>();
        pause = FindObjectOfType<PauseController>();
    }

    public void GameOver()
    {
        gameOverUI.SetActive(true);
        Time.timeScale = 0;
        playerShot.enabled = false;
        Cursor.visible = true;
        pause.enabled = false;

        if (fpsController != null)
        {
            fpsController.enabled = false;
        }

        currentCountdownTime = initialCountdownTime;
        StartCoroutine(RestartCountdown());
    }

    private IEnumerator RestartCountdown()
    {
        while (currentCountdownTime > 0)
        {
            restartCountdownText.text = Mathf.CeilToInt(currentCountdownTime).ToString();
            yield return null;

            currentCountdownTime -= Time.deltaTime;

            if (Input.GetKeyDown(KeyCode.Space))
            {
                currentCountdownTime -= 1;
            }
        }

        RestartGame();
    }

    public void RestartGame()
    {
        SceneManager.LoadScene(SceneManager.GetActiveScene().buildIndex);
        Time.timeScale = 1;
        playerShot.enabled = true;
        Cursor.visible = false;
        pause.enabled = true;

        if (fpsController != null)
        {
            fpsController.enabled = true;
        }

        restartCountdownText.text = "";
    }
}

Do it should to be like this?

@turbid notch Like this?

[turbid notch]

is there a reason your GameManager is DontDestroyOnLoad?

if not, the only change you need to make is stop making it DDOL