System Information
Operating system: Microsoft Windows 10 Pro version 10.0.19041
Graphics card: NVIDIA GeForce GTX 780

Blender Version
Broken: 2.90.1, 3e85bb34d0d7, master, 2020-09-23
Worked: ??? (I updated straight from 2.79 to 2.90.1, big jump)

I was working on a final version of an animation after completing the rough version, and I don't know when it happened but any new line strokes I drew were much MUCH thicker than they were. The thickness of the brush had not changed. And even if I made it as thin as possible, it was still thicker than existing lines.

I tried starting a new file to see if the problem persisted. It did not. It was only in that save file.

I then tried copying everything over to a new save file, and doing that fixed the issue. However I managed to figure out what had caused it in the first place, purely by chance:

Exact steps for others to reproduce the error

1. Open attached .blend file
   TalkingCat2.blend110 MBDownload
2. Go to Object Mode, and delete the "Stroke" object under Collection.
3. Go back to draw mode under either of the two remaining stroke objects, and draw a line.

(Hitting undo until before setting it to Object Mode fixes the issue)
(Doing anything in Object Mode also seems to cause the issue)