Just check it on your mobile phone.
Main thread always will be real time updates. Check below when it was last edited. You can check also on PC, just move your mouse pointer to subject. Whenever OP will edit title it will only effect on main post and new reply.
If you reply now your subject will same if you don't bother to edit.
When OP post
reserve that time topic title was
Open so it will never change even you change main post title.
That wasn't edited.
Summary is, when you will reply a thread your subject will same as current subject of OP. If OP edit subject it will not change your subject. But you can change only manually if you want.