I've been using Illustrator since 2016 and have always had this issue no matter what version of Illustrator I use. I've ignored it until now and would opt for the blob brush instead or just Pen tool it, but for this project, I unfortunately need the paintbrush tool and maybe its time to finally figure out what this issue is for once and for all. I'll try to be as descriptive as possible.
I am currently using Illustrator 2020 version 24.3. I'm on a pc using a Wacom 13HD Pen Display.
I select the Paintbrush Tool, set my brush profile to "Uniform" and select "Basic" from the Brush Library. With my Wacom pen, I draw a stroke and the line changes to the "Denim Seams" preset. I attempt the same with my mouse and the stroke, again, swaps to "Denim Seams". I can change the stroke to "Basic" like intended, but its pretty annoying to do this throughout the project. All I want is to simply draw a Basic line with the Paintbrush tool.
Things I have tried:
deleting "Denim Seams" - unfortunately I get a little stop sign when the paintbrush tool is selected so I can no longer use the paintbrush tool until I undo deleting Denim Seams.
I am thinking that somewhere along the lines perhaps you set up a preference that told the basic brush to use the denim seams. Then when you update to a new version, you say you want to import the preferences? So maybe you need to reset the preferences. That is the only possibility I can think that may cause this.