How to prevent pdf files from auto lock after editing?

If by "editing" the PDF file, you mean filling in a form you downloaded, then this is a known bug with the Preview app. The creator of the fillable form did secure it with a password to prevent the PDF core document from being modified, but allowed others to fill in the form. Unfortunately the Preview app has a bug where it takes the creator's password lock & applies it to the whole PDF once the Preview app makes an edit to the fillable form area.

Download a fresh copy of the PDF form and Use some other app to fill in the form.

This seems to be a known bug, and I've heard of similar issues before.

To be safe, it might be better to use a third-party PDF editor to update your form.

First, PDF files can not be edited unless you have an app which enables editing. Do you? If so, then the problem may be with the edit app and you should contact the vendor for assistance.