php: why form doesnt work when it’s in an include? [duplicate]

I don’t understand why any of my form doesn’t work if I appeal them in an include form

for example:

public function afficher_tache_veterinaire($id,$id_secteur){
        $emp=$this -> tab_employe();
        $e=$emp[$id];
        echo 'Bonjour employé numéro '.$id.": ".$e["fonction"]." ".$e["prenom"]." ".$e["nom"];
        echo ' vous etes affecté aux secteurs ';
        foreach($e["secteur"] as $sec){
            echo $sec["nom_secteur"].", ";
        }
        
        echo 'Que voulez-vous faire ?';
        ?>
                
        <!-- Bouton pour créer un médicament -->
        <form action="" method="post">
        <button type="submit" name="c_medicament">Créer un médicament</button>
        </form>
        <?php
        if (isset($_POST['c_medicament'])) {
            include 'c_medicament.php';
        }
    }


c_medicament.php :

<!DOCTYPE html>
<html lang="en">
<head>
<meta charset="UTF-8">
<meta name="viewport" content="width=device-width, initial-scale=1.0">
<title>Inscription</title>
</head>
<body>
<h1> création medicament</h1>
    <form method="post">
        <input type="text" id="nom" name="nom" placeholder="nom" /><br/><br/>
        <button type="submit" name="bouton" id="bouton">VALIDER </button>
    </form>

<?php 

if(isset($_POST["bouton"])){
    
    if (isset($_POST['nom'])) {
        $nom=$_POST['nom'];
        
        include_once "DB.php";
        global $bdd;
        $medicament = new medicament();
        $medicament -> creer_medicament($nom);  
        
    }else{
        ?> <script> window.alert("merci de rentrer toutes les valeurs ")</script> <?php
    }
    
}

?>

and creer_medicament($nom) is a basic INSERT INTO sql function.

my problem is that the function c_medicament work by itself but not when I include it in an other form,
it instantly disapear when i click on it.
I’m pretty confident that it’s something about session or a thing like that but I dont know what to do