Thanks to who have taken the time to respond.
What Hans says has clarified the matter for me. If I start a clean book writing codes it doesn't happen.
Now I have identified that the problem occurs to me after I add titles and sections, with the writing assistant palette.
Here I add an example, where I put titles and sections. Then I started writing a simple code
Solve [x + 1 == 0, x]
Then again when I put the = symbol it start to change the cell style..
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