Label widget created in function does not show in GUI

I have a register form consisting of two entries (password, confirm_password), a label for each entry, a button and a checkbox. I packed the creation of the widgets inside a register() function.

Now my Labels wont be represented on the GUI (entries, button and checkbox are also created inside the register() function) after packing all the register form related widgets inside said function (before that worked)

Here’s a snippet of my function:

root = Tk()
root.title(WINDOW_TITLE)
root.geometry("400x400")
root.minsize(400, 400)
root.maxsize(400, 400)
mainframe = ttk.Frame(root, padding=50)
mainframe.grid()

def register():
   # Create master password label
   create_master_label_text = StringVar()
   create_master_label_text.set(CREATE_MASTER_TEXT)
   create_master_label = ttk.Label(mainframe, textvariable=create_master_label_text)
   create_master_label.grid(column=0, row=0, pady=5)

   # Create master password entry
   create_master_entry_text = StringVar()
   create_master_entry = ttk.Entry(mainframe, width=50, textvariable=create_master_entry_text, show='*')
   create_master_entry.grid(column=0, row=1, pady=(0, 15))

   # Create master password CONFIRM label
   create_master_label_confirm_text = StringVar()
   create_master_label_confirm_text.set(CONFIRM_MASTER_TEXT)
   create_master_confirm_label = ttk.Label(mainframe, textvariable=create_master_label_confirm_text)
   create_master_confirm_label.grid(column=0, row=3, pady=5)

   # Create master password CONFIRM entry
   create_master_entry_confirm_text = StringVar()
   create_master_entry_confirm = ttk.Entry(mainframe, width=50, textvariable=create_master_entry_confirm_text, show='*')
   create_master_entry_confirm.grid(column=0, row=4)

   # Creater master password button
   create_master_button = ttk.Button(mainframe, text="Create Password", command=lambda: createMaster(create_master_entry_text.get(), create_master_entry_confirm_text.get()))
   create_master_button.grid(column=0, row=5, pady=(20, 0))

   cb_val = IntVar()
   cb_val.set(0)
   password_checkbox = ttk.Checkbutton(mainframe, variable=cb_val, text='Show password', command=lambda: ToggleShowPassword(cb_val.get(), create_master_entry, create_master_entry_confirm))
   password_checkbox.grid(column=0, row=6, pady=(20,0))
   
if firstUse == True:
   register()


root.mainloop()

If I move the labels out of the function, everything works again.

Thanks in advance