masks compression is just the regular compression that disables srgb. no benefit to using the masks compression generally.
try saving it as an alpha compression if its just a grayscale.
the compression for multi-channel packing basically packs/merges rgb together which results in quality loss. the only channel that does not have this is the alpha channel since thats basically a unique RGB channel altogether. it also doubles filesize when using alpha.
so its often better to not use alpha and combine the alpha channel texture with two others in a new rgb packed texture.