According to one source, the decision of reinstating the Saxe-Coburg-Gotha main royal arms happened after their recent visit at the ancestral Friedenstein Castle.
Royal Decree of 12 July 2019
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http://www.ejustice.just.fgov.be/eli/arrete/2019/07/12/2019030777/moniteur
I am not sure it is a good idea. I know the Royal Decree of 2015 says that the Princes and Princesses can use the titles they hold "by right of ancestry". Germany, however, is now a republic and the titles they are claiming, like Duke of Saxony and Prince of Saxe-Coburg-Gotha, are no longer legally recognized.
Apparently, the Belgian government said that, by ceasing to use the Wettin arms, the Belgian Royal House was being hostile to Germany and that wouldn't make sense today 100 years after the Great War. Quite frankly, that is a very odd statement, but, irrespective of suggesting how subservient some EU countries have become to Germany, I don't see how using titles that the Federal Republic of Germany doesn't recognize would be equivalent to extending an olive branch to your neighbors.
Besides, it is also inconsistent. Princess Astrid's children are Princes and Princesses of Belgium who descend in direct
female line from King Leopold I. Under Art. 4 of the Royal Decree of 2019, they will use then the new arms of the Princes or Princesses of Belgium who are not the heir, which include the Saxony arms. However, they are
not members of the House of Wettin and are not entitled by ancestry to use the titles of Duke of Saxony or Prince of Saxe-Coburg-Gotha as those do not descend in female line. To be consistent with the 2015 decree , they should use the arms of the House of Austria-Este instead.
Please also note that gender equality in the 2019 Royal Decree applies
only to the King or reigning Queen. For all other categories in the Royal House, including a monarch who has abdicated, the Duke of Brabant, a generic Prince of Belgium who is not the heir, and a generic Prince who is not a Prince of Belgium, there are differentiated shields for males and females in their respective coats of arms.
The best thing to come out of this Royal Decree in my opinion is that Princess Elisabeth at least got a proper coat of arms as Duchess of Brabant, differentiated from those of her siblings and cousins, which was badly needed. Besides, I personally think that all the proposed designs are quite nice.