Results of code module don't effect downstream previews.

Not just the last few versions - it has been like this since the code node was introduced :)
The reason is that the code node is a bit of an exceptional case, because it's the only node that needs to be compiled separately if it should ever have a preview. That's not to say that it's impossible, but it's harder

OK (although I don't really understand why all previews don't also need to have the appropriate sub-program compiled separately, or how the compilation would involve anything other than concatenating the relevant strings... clearly one of those things that seems simpler from the outside; if you say it's harder I'm sure it is).

"concatenating the relevant strings" is not as easy as it sounds, emphasis on relevant :)

Each node has their own shader at the moment, the code node is the only node where I can't predict what it will do beforehand, so it needs to pre-compile.

Yes... as I say, things always seem simpler from the outside, I do appreciate how it all gets much fiddlier when faced with the reality... :)