The issue of textures turning gray when "override for android" is checked, even if "read/write" is enabled, could be related to the handling of premultiplied alpha (PMA). When textures are exported with PMA, the RGB values are multiplied by the alpha value, which can cause unexpected visual results if not handled correctly in the rendering pipeline. It's important to ensure that the texture settings in your build environment, such as Unity, are configured to handle PMA correctly. This might involve disabling automatic PMA in the build settings or ensuring that the texture is not being premultiplied twice, especially for platforms like iOS or Android where such settings might be overridden.

All potential issues which cause white polygons after repacking can be found in the FAQ here:
https://esotericsoftware.com/spine-unity-download#spine-unity-4.3-beta
In your case it looks like compression format is causing troubles, as it can't be copied from block-compressed textures freely.

You would need to perform preparations as ensuring each attachment lies exactly on 4x4 texel borders to be copyable. The easiest solution is to set the format to uncompressed RGBA.