This question was raised yesterday as well relating to Vertical Design Rules and how they work with lines that start and end at the same location or on closed polygon shapes. Each line type has the ability to be Closed (in properties using AutoClose) or not closed (just that the first and last points of the line are in the same place (in 2D).
In the first release of TBC v5.20 we did have an issue with this in that the “special case” where the first and last point of a line are in the same location, when you apply different rules to the line, it could happen that the computed Z value for the first and last point of the line were different, thereby placing a flag on the location in the resulting surface model. We addressed that issue in one of the two patch updates for 5.20 so if you update to the latest patch (Check For Updates) then you should find that this works fine now.
Here is a video showing the results
I also captured a preview of the TBC v5.3 solution to this problem so you know that something good is coming down the pipe here
Alan