Text Style doesn't follow reformated text.
When I have a mix of text styles (say regular and bold weight of the same font) in a body of text, inserting new lines above the 'minority' weight causes it to 'lag' behind as the text is displaced.
This has the effect of moving the area that is in a different font weight backwards through the body of the text such that earlier words are now in that minority weight.
In the images attached note the style of the three phrases "Similar Solutions"; "Similar Problems"; and "Extended Logic", and the passage "freeist associations possible".
As line spaces are inserted the area of bold text retreats through the line such that the bold part of the last example reads "e freeist association possib" by the last image.
Just as an aside typing becomes EXTREMELY slow on files with more than a few artboards with text. Guessing this is to do with the re-rasterisation on the text on the fly.
AdminAsha B C (Software Engineer, Adobe) commented
Thank you for reporting. Can you please let me know whether this text is created in XD or copy pasted from some other application ? Which version of XD are you using ? Can you please share the file with us ?