Incorrect "Listpreamble" export to old version

[Daniel]

The new "Listpreamble" inset for numbered lists does not export 
correctly to older versions of LyX. In LyX 2.4.0dev, the listpreamble's 
content is inserted before the first item. Instead, when exporting to 
2.3.x, the content of the inset is just added at it's place in brackets 
"{}". This leads to a different and incorrect output in 2.3.x.

I am wondering whether this is a mistake or the way it should be. I came 
across this while trying to create a lyx2lyx function for a front-, 
main- and backmatter function (https://www.lyx.org/trac/ticket/12424).

I struggle to get the correct export to LyX 2.3.x but maybe that is 
acceptable given the "listpreamble" does the same.

Daniel