Not only is that weird, but it is impossible. If you are changing the value of a field in the portal, that change is being made to data in your table and it will not magically revert to the original value.
1 You can prevent the user from editing the field by denying browse mode access via the behavior settings in the Inspector.
2. I suspect that the field is not from the parent table, but from a different table such as a join table and thus, when you go look for the change made in the portal, you are not looking in the correct place. Such changes will be saved automatically unless you have deliberately set up a script trigger controlled script to automatically revert the data in some way.
You were right, I thought I was referencing one thing, but was in fact referencing another.