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:
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.
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"I select the Paintbrush Tool, set my brush profile to "Uniform" and select "Basic" from the Brush Library."
"Basic" is NOT a brush. "Basic" deletes an exsting brush from the path.
The paintbrush tool will always use a brush when drawing. Automatically.
If you want to draw without a brush, then you MUST use the pencil tool instead of the brush tool.
I'd just like to drop a thanks here. I was having a similar problem, I think, to the OP and this post solved it for me.