If a map uses a BOOM specific special to support different behaviours of a sector depending on the triggering line, the tag-based link between the trigger line and the sector appears to be broken even though the map editor is opened in Boom mode.
Secret sector #4 is triggered by its four surrounding lines from the inside, and by a fifth line (#2600) with a switch. This line uses a special which makes the sector lower in a slower speed than the DOOM default and wait for 5 seconds before rising again.
Targeting this line does not highlight the same-tagged sector.
Targeting the sector does not highlight this same-tagged line, only the four surrounding ones.