snappyHexMesh does not extrude (using medialAxis) in thin gaps
The medialAxis shrinker by default used to disable edges with any point on an extruded wall to contain the medial axis. Now this is a bit more intelligent and it only checks the actual medial axis point to be away from the wall.
// Revert to <=1806 behaviour: disableWallEdges true;
Below is a hacked version of the addLayersToFaceZones tutorials where the 'B' faceZone has been shifted to be one cell to the right of the 'A' faceZone.
1806 (or with