Grayscale Texture have color noise?

Hello everyone, my English is not well, so I try my best to express my problem.

As you can see, when I import the same image into the UE, I get inexplicable color noise. I suspect it is because the UE has performed a compression at the time of import, maybe not?
The only solution to this problem is to set the “compression settings” to “grayscale”, but this will lose the information of other channels.
At some case, I do not want to put too many textures in the shader and I’m worried about the burden of doing the math, so I’ll use the same alphamap to make the difference in the RGBA channel and use it independently in shader Each channel.
All in all, is there a way to solve this problem?
Thanks!!