Based on your description of the problem, it sounds like you have an auto replace setting causing the issue rather than a visible text boundary as suggested by G.O.Tuhls

I would ask whether this happens every time you press return at the end of a paragraph or only in special cases when the preceding content is particular text. Does it happen when you put your cursor somewhere earlier in the paragraph and press Enter?

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Andrew Lockton
Chrysalis Design, Melbourne Australia