Hi @Anonymous ,
Do you mean that your display form will change to first item or not update?
I've made a similar test but not met the same problem like your description.
Here's my test for your reference:
in my test, gallery in screen3, display form in screen4, edit form in screen5
1)In screen3
insert a gallery, set its data source to your list
set its arrow button's OnSelect:
Navigate(Screen4)

2)in screen4
insert a display form, two buttons, one for backing to home, one for jumping to edit form
Set the display form's data source to your list
Set the display form's Item: Gallery2.Selected
Set the home button's OnSelect:
Navigate(Screen3)
set the edit button's OnSelect:
EditForm(Form1);Navigate(Screen5)

3)in screen5
insert an edit form , two buttons, one for submitting, one for backing to display form
Set the edit form's data source to your list
set the edit form's Item:
Gallery2.Selected
set the submit button's OnSelect:
SubmitForm(Form1)
set the back button's OnSelect:
Back()

What's more, please check whether you've updated successfully.
You could also add refresh function in back button.Try this:
Refresh(listname);Back()
Best regards,