Service Area polygon appears to fill in between the roads for areas it should not reach

Question

In ArcMap 10.5.1, I have created a service area polygon of 1000m along a road network, with a trim of 50m. In the image you will see where the road is linked around, the inside is filled in (red circles). However there should be a hole in this area as it is outside of the trim and not reachable within the parameters.

I am using ArcGIS Network Analyst. After adding the Network Dataset I used the Network Analyst toolbar to create a new service area. I added Facilities, which was one single central point. Within Service Area layer properties I set the Impedance to 1000m, in the direction away from the facility. I chose to Generate Polygons - Detailed with a 50m trim. I also chose to generate a line.

What am I doing wrong here?

Answer 1

See this bug description: Bug NIM066645

Network Analyst does not trim a Service Area polygon properly if the area is enclosed by a Network Analyst Route in versions 9.3.1 SP2 and 10.0 SP1. If trim option is only meant to trim the exterior of the service area, then please update the documentation to reflect this behavior.

So this seems to be the expected behaviour, but the documentation is not crystal clear (is 'periphery' only the outer border of the service area or does it also include 'islands').