It was, and is still called KQ8 by Roberta Williams as well. She was as official as you can go when it came ot the old Sierra games.
There is no such thing as 'semi-official', there is just 'official'. Also these are the descriptions that appear after the game's release as well, btw. They exist both before and after the game's release. Infact, the game was marketed as both King's Quest: Mask of Eternity (title) and KQ8 (abbreviation) and King's Quest VII/King's Quest 8 (description) both before and after the release. This is a fact. Infact, Sierra released initial copies in the US with a gold seal that listed either King's Quest 8, or it being the "Eighth game of the Series".
You can see the seal here, but its unreadable (unfortunately), but I had it on my original box (which I unfortunately no longer possess after several moves);
There were even releases of the game patches to fix the bugs in the game that had KQ8 in the file description.
It was never considered a 'spinoff' to think otherwise is living a to lie to onceself see 1984's doublespeak'
Ignoranz (ingorance) in the German definition: Someone's conscious decision to ignore the facts - usually that's a facet of arrogance (or, less often, stupidity). It's more a description of attitude than state. Not knowing is only the result of it. In any case, you cannot be innocent of "Ignoranz".
Yes, King's Quest: Mask of Eternity is the official box title, but KQ8, KQVIII or King's Quest VII, or 8, or "eighth game of the series" are official 'abbreviations" or descriptions for the game.
It's no different than Quest For Glory: Shadows of Darkness having the abbreviation QFG4 or described as Quest For Glory IV: Shadows of Darkness, or Police Quest: Open Season having the abbreviation PQ4 or description Police Quest IV: Open Season.
Or Lost Secret of the Rainforest, having the abbreviation EQ2 or described as EcoQuest 2...
Also remember The Beast Within: A Gabriel Knight Mystery, has the abbreviation GK2 or description of Gabriel Knight 2...
The two Laura Bow games The Colonel's Bequest, and The Dagger of Amon Ra, have the abbreviations of LB1 and LB2 or descriptions of Laura Bow 1 or Laura Bow 2.
Leisure Suit Larry series did this as well the second game is called, "Leisure Suit Larry Goes Looking for Love (in Several Wrong Places)", but is officially abbreviated as LSL2, or described as Leisure Suit Larry 2.
You can go on and on with many of the Sierra series leaving out the 'numeral' in at least one sequel for whatever reasons.
Sierra did this sort of thing in many series. The reasoning behind it is unknown.
You can find it in modern series as well;
One modern example by Activision. Call of Duty: Modern Warfare 2 is also just called Modern Warfare 2 in the title screen of the game. The series went back to calling the third game Call of Duty: MW3 (Modern Warfare 3)
Enterprise (later Star Trek: Enterprise) still a Star Trek series, even though they chose to leave off the IP title early in the series. All seasons DVD releases were later remarketed as Star Trek: Enterprise to clarify things.