Outlines changing when using brush tool

I am trying to create a layered drawing in illustrator on my ipad, with shapes in different colour fills + some with, some without outlines. When I draw a new brush stroke (in an outline, no fill), it changes the outline of other paths (on locked/other layers) to have the same outline colour/thickness, where they had no outline previously... its very frustrating. Has anyone experienced this / is there a work around? Beginning to think I should move to ProCreate.