paragraphs desappear from page

I can’t create paragraphs on page.

The line are allways rebuilt and <p> erease.

For example, when i write :

line 1

line 2

in back-office.

The result in front-office is

line 1 line 2

And if I add <p> in text tab from page, each time I save, <p> desapear too.

Example in back-office text -ab

<p>
line 1
</p>
<p>
line 2
</p>

become after saving, as soon as i clic on visual tab :

line 1

line 2

And

line 1 line 2

in front-office

I have desactived all plugins

What can I do ?