I would love to tell you what happens, but I would need your file. What material system have you used, what image format, psd or exr, perhaps jpg or png. There are a lot of options and combinations that can lead to a result like this.
Please share a project file; if you like a precise answer, use “Save Project with Assets…” in case of textures. If it is too large to be attached here, I’m happy to set up an upload link. (Dropbox, Apple, Amazon, Google, Wetransfer, Adobe are the cloud-services I touch, please no shortened URLs, thanks)
The other question is, why do you use a polygon selection? You can just place the texture there; see the file below.
It becomes a little bit more work if you like to see the brick bump-channel under the Graffiti. Also included in the file.
Please have a look at the attached scene file.
More (scroll down)
https://help.maxon.net/r23/en-us/Default.htm#html/TTEXTURE-ID_TAGPROPERTIES.html?Highlight=alpha + Decal
All the best