Seems wrong to me. Setting the offset to -90Degree works for me, but the result is different from what is shown by the ConstraintComponent.
It is also very difficult to understand what is rotating around what, cause rotating the constraint round its axis does not always affect the result. Taking the door as example, rotating the constraint around its Z-Axis does not change anything.
It seems the pivot of the door is rotating around the location of the constraint. The X-Axis of the constraint represents the connection of the constraint to the object. Is that correct?
Thanks for the report. I can see how that can be confusing. I have created JIRA UE-39897 for the issue and our developers will be investigating further. You can follow that link to monitor the status of the issue and please feel free to vote.