Hello!
I encountered a strange situation when importing textures to UE4.23 yesterday: a specific area of my basecolor, an area that had some rust/dirt that was caused by a specific Painter layer, would become “low res” and the colors would change from a brown to purple, almost petroleum-like hues.
These textures were exported as .tga and I tried disabling/enabling dithering, I tried .png 8 and 16 bit, dithering on/off…
I backtracked the problem to the layer that created the rust effect. I was experimenting a bit so pardon me if the way my layer was setup makes no sense but it was as follows:
-A Fill layer with a brownish color. The layer mask had a Edges Damage on it with a slope blur Filter and I added a Paint effect to customize the damage. I had a fractal sum 2 as custom noise on the Slope blur filter…but i tried other ones like Cloud to no avail. I tried playing with the settings EXTENSIVELY for 2-3 hours.
If I left this layer active, it would the create the artifacts after saving the newly uploaded texture to Unreal. I noticed that the Defer Compression until saved option was checked in these cases. This option is in the Texture’s Details I believe.
Here are the pictures of the desired effect and the problem that occurs in engine.
I’ve since decided to simply delete this layer and recreate the effect without using a slope blur. Im getting the feeling there is something specifically mask related in this case but i dont know what, since it appears OK upon import to unreal and then distorts after save.
Quite the minor mystery…Any ideas as to the cause would be greatly appreciated! 
Cheers!