When trying to create a widget with specific steps, the drawing order may not be correct. This is not reproduced when Global Invalidation is disabled, and the state can be restored. This issue is probably related to the ZOrder.
The CVar `Slate.InvalidationList.EnableReindexLayerId` set to 1 fix the bug.
You can confirm the fix with the command: "Slate.VerifyWidgetLayerId 1". The command will warn you when the bug occurs and auto deactivate when detected. It's an ensure, it may just send a silent crash report depending of how you are setup.
"Dialog" widget will appear in front of the "Menu" widget.
"Menu" widget will appear in front of the "Dialog" widget.
There's no existing public thread on this issue, so head over to Questions & Answers just mention UE-136046 in the post.