How to increase the size of the mdtextfield inside a BoxLayout --kivy

I want to create a Textfield with a particular size_hint and once the text inside the field reaches a particular minimum height than the Textfield size_hint will be set to None and y_scroll will get active..Seems perfect when using FloatLayout. But when I am using a Boxlayout the size of the MDTextField seems not to change . if I change it's size_hint to suppose 0.7 then the textfield Having mode set to rectangle will become a line and if keep it to None and changes its height via height property the size doesn't increase.. How can I fix the problem .. Any solution except for changing the layout because only BoxLayout is the thing which I want/Use

my code::

str = '''
    
MDCard:
    pos_hint : {'x':0.056 , 'top':0.836}
    size_hint : 0.88,0.82
    
    ScrollView:
        do_scroll_x : False
        BoxLayout:
            height : self.minimum_height
            orientation : 'vertical'
            size_hint : 1,None
            spacing : 15
            padding : 55
            
            MDTextField:
                id : input_field_1
                mode : 'rectangle'
                size_hint : None,None
                height : 500            # setting height doesn't seems to work either
                #y_scroll : True if self.minimum_height > 400 else False
                multiline : True

            MDTextField:
                id : input_field_2
                mode : 'rectangle'
                size_hint : None,0.7 # see the result 
                #y_scroll : True
                multiline : True

'''
class My_4app(MDApp):
    def build(self):
        return Builder.load_string(str)
My_4app().run()

Any help is quite appreciated