I may be too late though...
@UzukiCheverie had mentioned that pen size in Flash adjusted as you zoom and Clip Studio had that option too
I think that may be the problem... cause I experienced one when creating my own brush
You can choose not to have the brush size adjusted to zoom by accessing setting of brush called sub tool detail (located at the down right corner of tool property box)
If I'm not mistaken, you can uncheck 'specify by size' or something
Uhh, I'm not on my computer right now so maybe it's not accurate
If it's not the problem, I'll think up for some other solution
Hope it helps!