Hi friends, I wrote a guide showing step by step process for channel packing in gimp.
https://toufique-azad.blogspot.com/2023/03/how-to-channel-pack-textures-in-gimp-210.html
finally, the problem is solved. I was using latest drivers from AMD but i had been using their "PRO" edition before.
Now, i installed their "Adrenalin" edition of the driver and the glitch is gone.
i tried that. i increased upto 4x but no use.
i played around more, i changed to decal shader from diff+normal to just diff and the glitch is gone.
I think the problem is in shader code, im not sure.