Since we switched to DX12 we started to see strange behavior in material previews. I was able to reproduce the problem in UE 4.19 vanilla as well.
We usually have to force recompiling the shaders to fix the issue. I was unable to reproduce this bug using previous version of DX
See attached video. I have reproduced this issue in 4.18.3 and 4.19.2.
Result: After previewing nodes a few times each, the preview will start to behave unexpectedly. TexCoord preview might show SphereMask result, 1-X node preview might show TexCoord result or Rotator result, etc.
There's no existing public thread on this issue, so head over toAnswerHub just mention UE-59204 in the post.