Have you seen this?
Coming up the last beat on starboard opposing boats are sufficiently separated that the leeward/ clear ahead boat thinks he has room to tack and duck the covering boat. The problem is that, as he luffs to begin his maneuver, the covering boat bears away and closes the gap.
The problem for the umpires is that until the leeward/clear ahead boat passes head to wind he is the right of way boat. This means that the other (covering) boat is not restricted by Rule 15. Immediately the leeward/clear ahead boat passes head to wind he becomes keep clear boat and at this point the covering boat is restricted and, since the tacking boat intends to pass astern 16.2 applies. Therefore the change of course to shut the door has to be timed perfectly and a steady course held as soon as the other boat passes head to wind.
The principals are pretty much the same as Call D4. The problems of judging it are also pretty similar In keelboats you might be able to have the umpire call "Tacking" call as the boat passes head to wind and "Holding" for the other boat - but in Fireflies and other dinghies it's probably too fast for such a conversation.
I would say it is a high risk move. The sailor may know what he is doing but the Umpires need to be sure. A call against the boat trying to close the door usually costs that team the race. They are probably in a tight situation anyway, to be trying to hold a boat out to the left. So the potential for upsetting the sailor is pretty high.
I don't think the Umpires get the call wrong - they simply have a very difficult judgement to make depending on split second timing and, while the sailor knows what he is trying to achieve, it may not be what the umpire sees.