Issue while walking on multiple box collider 2D

Hi !
I've been making a terraria-like game this afternoon where the level is make of blocks and I've encounter an issue when my player start to hit a certain speed, or when he change his direction, like in this video I made (start at 0:30).

I think the player's collider (a sphere) and a block's collider (a cube) are blocking each other but I don't understand why it happen so randomly, since all the blocks are at the exact some Y position, it should be a perfectly flat floor.

Here's the player controller I've been using :

 using UnityEngine;
 using System.Collections;
 
 public class Player_Controller : MonoBehaviour {
     
     public float speed;
     private Rigidbody2D rb;
 
     void Start(){
         rb = GetComponent<Rigidbody2D>();
     }
 
     void FixedUpdate (){
         float moveHorizontal = Input.GetAxis ("Horizontal");
         rb.velocity = new Vector2 (speed*moveHorizontal,rb.velocity.y);
     }
 }

And here's some details :
The player
A cube
The scene

Thanks for your answer !

ps : Sorry if there is any misspell, english is not my primary language :)