hello everyone , i find out a bug in materials in unreal ue4.20 as you can see in those images :
https://forums.unrealengine.com/core/image/gif;base64
here i just added texture node and compile is more then 400 at start
https://forums.unrealengine.com/core/image/gif;base64
here i just create simple material and change privew many times
after i lot of tests i finded out this
and i tryed to create project to test if this my problem and its broken just like my project
epic please FIX