Apparently, this is indeed a Scintilla issue. I found via Google search, a feature request thread titled “Fold { on relevant line”.
https://sourceforge.net/p/scintilla/feature-requests/8/?page=0
Note that this request is request number eight (8) and the date is fifteen (15) years ago! When an open request in a list currently numbering in four digits is numbered with a single digit, you can probably deduce no solution is likely anytime soon.
Seems to me the only general solution for scintilla is to trigger fold start on the list of “foldable” language structures, and base fold ends on a multiline regex pattern that encompasses the language syntax definition of the structure. (That is, the one the language parser understands)