Line thickness changes when moving object or copy & pasting

Hello everyone,

My illustrator worked fine for months & out of nowhere a very annoying bug appeared (or maybe I pressed something accidently).

Whenever I move an object (drag, allign or copy/paste): The thickness of the lines is changing (and stays that way even if I use the un-do shortcut).

Here is an example, before and after moving:

As you can see the line in the red dotted circle got thinner.

Here is a video recording for better overview:

Does anyone know how I can fix this? I already tried to re-install everything, reset all presets. But it keeps happening in that file. Even if I copy/paste the contents to a new illustrator file.

I use the latest version of Illustrator from the creative cloud subscription.

Thank you in advance!