Dear Mahammad,
You shouldn't be worried about it. When you write the posting date at first, system captured it as baseline date and fill the "Baseline Date" field. If you change the "posting date" for second time, after filling the baseline date field automatically, the baseline date does not change automatically anymore.
The "Baseline Date" field is filled automatically by the system when this field is empty. If this field is not empty, system does not change because system can't know this field is filled by you or by system before.
In short, there is nothing wrong with your baseline process according to your story.
Sincerely,
Beste