Hi. Sorry if this has been reported before.
Just tested this in Aseprite 1.2.9 (Latest available on Humble Bundle. Not tested in beta version.)
I want to use a custom brush with rough “noise” on it, to add texture to a drawing. I had hoped to use the Lock Alpha mode, to allow me to just paint over the image, adding texture to areas that are already painted. This works for a normal brush, but for a custom brush (created with ctrl-b) the alpha locking feature does not seem to work, and the brush is applied to the transparent areas too.
I can work around this by using the magic want tool, but its quite a bit of extra work, especially if an area is already non-uniformly colored.
While I’m here, I might mention that some kind of “noise brush” would be an awesome feature for doing something similar. It could just randomly offset the value of each pixel. Thanks! 
EDIT: Ah, I realised that the jumble tool is basically what I was thinking of. Great, that already exists