New face affects close by faces I try to remodel a asset store asset. I have used the project knife  tool and now I try to add new faces to that cut out. I select four vetices and press F but it affects the UV of close by faces, see screenshot

Before

After


Thats a backface and shouldnt have UV at all.

edit: Ok so I followed this tutorial to get better topology
https://www.youtube.com/watch?v=axCYMBRMXo0

It now looks better, but there is some strange changes here is the original model in unity, see how the reflection is applied to the PBR material


Now after my changes


My typology now looks like this
What you are seeing is Blender's smooth shading view trying to smooth out the appearance of the faces. Whenever you add new faces, you give Blender more faces to try and shade smooth. Before adding the face, the original face was an edge. Now you are telling Blender that it is actually a corner, and thus the shading is updated. Now, the other reason for how the shading looks is that the original face is an ngon, so it has more than 4 faces. Ngons are much more prone to shading issues, as they have to be triangulated before rendering, and this can lead to unexpected behavior. Especially in this case, you should try to avoid them. Look into trying to clean that up and make your topology better. There are plenty of resources out there for that, both on this site and elsewhere.