Didn't the 'Saxe-Coburg' decision take place in 2015? So after Amedeo's marriage and before Maria Laura's? (or am I mistaken in my timeline) Could that be (part of) the reason for the difference. That since the royal family now has an established surname other than 'de Belgique', the king intends to use those surnames - which will be 'van/de Saksen/Saxe-Coburg' for the children of himself and Laurent and 'd'Austriche-Este (Habsburg-Loraine)' or indeed 'van Oostenrijk Este (Habsburg-Lotharingen)' for the children of Astrid and Lorenz.
Both decrees were gazetted on November 24, 2015 with effect from November 12, 2015.
Royal decree reforming and clarifying the titles and surnames of the royal family:
https://www.ejustice.just.fgov.be/c..._date=2015-11-24&numac=2015021075&caller=list
Royal decree granting retroactive consent to the (2014) marriage of Prince Amedeo:
https://www.ejustice.just.fgov.be/c..._date=2015-11-24&numac=2015021074&caller=list
Legally, the change (although the King would claim it is a "clarification" and not a change) was already made in 2015 when King Philippe issued a royal decree on the matter.
I've written many lengthy posts about it in this thread and will try to find and repost them again later.
To begin with, here is a repost of
my earlier post on why "of Belgium" was popularly perceived, between 1891 and 2015, as the surname of the royal family.
[...]
1. Belgian concepts of law
Substantive information and evidence for how Belgian law interprets surnames and titles
is set out in Post 2. But in short, in Belgium, a member of the nobility such as Princess Alix of Ligne is not technically a Princess of Ligne as no such title is legally registered. In accordance with Belgian law, she has the title Princess and the surname "of Ligne".
If the law should treat the titles and surnames of Princess Astrid of Belgium in the same way as those of Princess Alix of Ligne, Astrid would legally have the title Princess and the surname "of Belgium".
2. No other reasonable alternative for male-line royal princesses/princes born after 1921
Duchess/Duke of Saxony and Princess/Prince of Saxe-Coburg-Gotha were excluded from the documents of descendants of Leopold I born after April 1921 (refer to Post 4), as shown in for instance
the marriage act of Prince Laurent, where his name was listed as
Zijne Koninklijke Hoogheid Prins Laurent Benoît Baudouin Marie, Prins van België
Son Altesse Royale le Prince Laurent Benoît Baudouin Marie, Prince de Belgique
(His Royal Highness Prince Laurent Benoît Baudouin Marie, Prince of Belgium)
The Civil Code stipulates all citizens should mention their surname in official records. (I will elaborate on this requirement in another post.) It has no exemption in the case of the royal family.
Due to this rule, Laurent could not be surnameless in his marriage act, which as quoted above names him as "His Royal Highness Prince Laurent Benoît Baudouin Marie, Prince of Belgium". If the legal rules were followed, his surname should be someplace within "His Royal Highness Prince Laurent Benoît Baudouin Marie, Prince of Belgium".
His Royal Highness, Laurent Benoît Baudouin Marie, and Prince are evidently not surnames. With the other alternatives exhausted, his surname has to be "of Belgium".
As Princess Esmeralda once said in an interview:
Vous vous appelez « de Belgique » ?
C’est ce qui est écrit sur mon passeport. Nous n’avons pas d’autre nom, puisque Saxe Cobourg a été abandonné avant la Première Guerre mondiale par mon grand-père.
Your name is "of Belgium"?
That's what is written in my passport. We have no other surname, since Saxe Coburg was abandoned before [sic] the First World War by my grandfather.
3. Original meaning of the 1891 Royal Decree
The Government that passed the Royal Decree of 1891 (discussed in Post 4) understood "of Belgium" to be a surname.
The official report included with the decree stated:
Après 60 ans d’une vie nationale à laquelle la maison royale est si indissolublement liée, ses membres ne portent ni nom ni titre qui les rattache directement au pays.
Dans le sein de la grande famille belge où, à tous les titres, ils sont les premiers, on ne les désigne que par leur prénom, mais, à l'étranger, on qualifie nos princes et nos princesses du nom de la Belgique et tel est bien le nom qui leur revient.
Translation:
After 60 years of a national life to which the royal house is indissolubly bound, its members carry neither surname nor title that attaches them directly to the country.
Within the great Belgian family where, of all the titles, they are paramount, they are referred to by only their forename, but abroad, our princes and our princesses are designated by the name of Belgium and that is the surname to which they are entitled.
Le Soir also
raises the point that this is the only legal text prior to 2015 to specifically mention the "surname" of the royal family.
4. Recognition in personal documents
"Of Belgium" has appeared in the surname box in blood princesses'/princes' birth acts and identity cards.
Leur acte de naissance ou carte d’identité porte donc le nom « de Belgique ». [...]
« Dans les actes de naissance, nous confirme-t-on à bonne source, c’est le titre “prince de Belgique,” qui apparaît dans la case “nom’. On considère donc que le nom est “de Belgique,” quand on a en même temps le titre “prince de Belgique”. [...] »
Translation:
(Their birth act or identification card thus bears the surname "of Belgium". [...]
"In the birth acts," a reliable source confirmed to us, "it is the title 'prince of Belgium' which appears in the box 'surname'. So we consider the surname to be 'of Belgium' when they simultaneously have the title 'prince of Belgium'. [...]")