@ with its document-meaning of character code 12 is useful in preambles, especially when dealing with TikZ or beamer overlays. But when the preamble of any included document contains such an
@ character, using the subpreambles option to the standalone package causes the included document to be permanently omitted. The package reports that the preamble had changed also when it in fact has not.
\makeatletter at any point in the main document preamble or in the included document preamble makes it work again, but only as long as it is never reverted with a
\makeatother, even when the latter is first issued inside the document environment.
An example is attached. It contains both main and included file via filecontents. I suppose this use case will work with any measure introducing Latex3 compatibility, but enabling the
@ character may perhaps be possible by easier means.