Variation in line weight changes when I edit paintbrush line

Hi

I have an Illustrator file made in an older version (maybe 4-5 years back in time).

I started working on it again in the 2023 version of Illustrator. In the file I had made a lot of paintbrush lines. They were curved and automatically it appeared a nice effect: The thickness of the lines differed throughout the line. (Calligraphy effect).

When I started to work with the file in 2023 version, I saw that the same type of lines get much less variation in thickness. Even the lines I had allready made in the older version, changed, if I edited them in some way. (For instance I changed the color of some lines, and it made them loose the original variation in thickness!). If I make a copy of one of these lines and paste it, the copy is also different from the original. 

What is happening? Doesn't this effect exist in newer versions? I see a small difference in thickness, but much less. How can I keep my original lines and at the same time be able to edit the color or the length? How can I make new similar lines?

I allready tried to open the file in 2024 version (suggetion from the support team, by the way, they haven't been able to give me a good support, thats why I write here). Didn't help.

See screenshot of original line (how I want it to be) in top of the screen shot, and the same line copied in the bottom.

Thanks, Stine