[Gimp-user] Bizarre Selection-Filling Issue

[Shiizumi <forums gimpusers com>]

I'm attempting to manually colour a previously black-and-white image with Gimp.
Several sporadic many hours (over the course of a few months) I'm nearly done,
and wanted to re-shade the hair.
I take my previous hair-shade layer, do Alpha To Selection to select all of it,
make a new transparent layer, and with the Free Selection tool, add in more to
the selection.
All seems good until I attempt to colour it! The layer is transparent, and I was
using Brush tool set to a shade of grey, and while grey shows up...not all the
time, and sometimes outside the borders. (Screenshot 1; my attempt to colour in
the selection. On the left is the Selection Editor, to make sure Gimp actually
knows the selection is there.)
Essentially, Gimp is attempting to shade in the new layer just like the old one.
I've tried this before, and have never had this issue, so I'm not sure what's
wrong with it. (Making new layers, applying masks, deleting the old shading
layer, etc. didn't fix this problem. Additionally, in the Selection Editor
screen, I can click the button to draw along the edge of the selection, and that
works perfectly fine.)
-
Gimp Version 2.8.4; Windows 8.1 64-bit
I don't recall fiddling with settings, but I'll assume something isn't default.

Attachments:
* http://www.gimpusers.com/system/attachments/228/original/Screenshot_(82).png

-- 
Shiizumi (via www.gimpusers.com/forums)