To be more accurate:

• [[Category:One two three four]] has been replaced by [[Category:One two three four|Category:One two]] and [[Category:One two three four| three four]] (note the "Category" in the sort value, and the space at the beginning of the second sort value)
• Other categories are in between the 2 categories, and everything is put on a single line with the portals

Sounds like T76715: Same Category get added multiple times ,for every change in “Sort this page by default as” value.

Not sure, for several reasons :

• I don't see any mention in T76715 of categories becoming grouped on one line
• I tried reproducing T76715 and I see the category appearing several times in the dialog box, but not in the actual wikitext diff
• The steps described in T6715 start by creating a new category, there's no new category in this problem

I think that the cats getting grouped on one line is a red herring (a
second, unrelated bug). If memory serves, that happens when someone tries
to delete the "unnecessary" blank line at the end of the article.

I agree with the previous comment -the issue seems similar to T76998 and T85671. The categories are not deleted, but "jammed" together on one line - the diff shows it.

{{Portail|Savoie|communes de France}}[[‪Catégorie:Villes et villages fleuris‬|‪Catégorie:Villes et]][[Catégorie:Commune de la Savoie|Crest-Voland]][[Catégorie:Station de sports d'hiver en Savoie|Crest-Voland]][[‪Catégorie:Villes et villages fleuris‬| villages fleuris‬]]

Also I agree that the bug describes a different issue - spontaneous(?) addition of sorting order.
Although the bug is not reproducible, I'd like someone to take a look at it, so assigning a priority as Normal.