Paragraph with weird text spacing behaviour

I have some trouble with a paragraph.

I am using Styles & Formatting, having set up about 5 styles. All worked well, until i added and inherited one new style from an old one. Suddenly now the old style will lead to the text spacing changing while i type. I don’t know how to better explain or describe this – it looks as if the text line will grow (in spacing in between letters and words) pixel by pixel, while i am typing.

Please note: alignment is set to left.

I tried removing the style, adding it again: same problem.
I tried removing the containing table, adding it again anew – same problem.

My mind is boggled by this. Can someone tell me what this is and how to turn if off?

Thank you!