Render from all my dynamic SelectField in my FieldList are overwritten by the first one

Question

In my project, I want to create several pair of Select field to make a composition of different materials (on selection among all the label of the material, and one selection among 0.1 and 20 %). This is my forms :

class MatForm(Form):
    mat = SelectField()

class List_Compose_Form(Form):
    list_mat = FieldList(FormField(MatForm))
    submit=SubmitField('Select Composition')

So I create a function to create a list of Mat form with default value for all the key like ('Batch(MAT)' or 'MAT (%)') and all the associated possible choices (list of possible labels or a range between 0.1 and 20 :

def compose_form(grades):

raw_name = 'interface_app\grades_raw\\'+str(grades)+'_clnd.raw'
[dic_compose,dic_choice] = pickle.load(open(raw_name,'rb'))
all_mat_items = []
for key, values in dic_compose.items():
    mat_id = uuid.uuid1()
    mat_entry = MatForm(request.form)
    mat_entry.mat.label = key
    mat_entry.id = mat_id
    if re.search('%',str(key)) :
        choice = [(values,values)]
        choice += [(str(i/10),str(i/10)) for i in range(1,200)]
    else: 
        choice = [(values, values)]
        choice += [(dic_choice[key][i],dic_choice[key][i]) for i in range(1,len(dic_choice[key]))]
    mat_entry.mat.choices = choice
    all_mat_items.append(mat_entry)
return(all_mat_items)

This part is actually working and I can indeed create my composition: composition page

This is the app where I create the list and I save the data in the session

@app.route("/compose",methods=['GET','POST']) 
@grade_required
def compose():
   the_grade=session['grade']
   session['dic_compos'] = {}
   form =List_Compose_Form(request.form)
   list_mat = compose_form(the_grade) 
   form.list_mat = list_mat
   if form.submit.data and form.validate():
       for k in form.list_mat :
           k.validate()
           session['dic_compos'].update({k.mat.label : k.mat.data})
       return redirect(url_for('test'))
   return render_template('compose.html',form=form)

But for some unknown reason the data that I get from each field is overwritten by the first like : session dictionary after composition

For me this issue is probably linked to MatForm(request.form) in my loop of composition or probably to my jinja2 template :

<div class="table-responsive-sm">
  <form action="" method="post" role="form">
    <table id="data" class="table table-dark table-responsive overflow-auto" style="height:400px">
      <thead >
        <tr>
          <th>Labels</th>
          <th>Selected</th>
        </tr>
      </thead>
      <tbody>
        {% for mat_form in form.list_mat %}
          <tr>
            <th>{{mat_form.mat.label}}</th>
            <th>{{mat_form.mat}}</th> 
          </tr>
        {% endfor %}
      </form>
    </tbody>
    </table>
    <div>
      {{ form.submit }}
    </div>
  </form>

And I tried to add csrf token or to used render_field from (_formhelpers.html )with different name to avoid overwriting but nothing works. I'm working one this issue since 2 days, so every help is welcomed.

Tell me if you need additional precision/explanation this is my first ASK.