Created a combinator with the following code:

on_nth_tick[60] = function(event)

end

Produces this blueprint string when blueprinted (or copied / cut):

0eNqllFlqwzAQhu8yzypoX3yVEoydqK3AloMih4bgu1d2wDKFkCC/CCRmvlk089+h7UZ7Ds5HqO7gjoO/QPV5h4v79k03v8Xb2UIFLtoeEPimn2/d2Hwch751volDYDAhcP5kf6EiE3rHOznXa+j61MRmw6DTAYH10UVnH+ksl1vtx761IQV5gUJwHi7Je/BzDomIEdzSmUKkCmMYurq1P83VDWE2+HJdtOFJ4VcX4phe1pAPiw8MC22cO8f5i/rfgRAtuRGaKLxpRDHMCMYM5lyuMFYOU9JQxqnOmfFimCHESGwYWVmiPDFMhWCE6Nx/uaP/CaWVJLllas9nGqIpFrlMveMztWQcG5XLNMUwaQQ3khqWRxYXwwTTRkkqc8vIjgWQOi2A5HRDK9+APKuETYdp5vxTEfpUzp7qxyxMi5ZVG+FEcE3qsRgrhtMYqbQpZJr+AJRFt4Y=

Once the combinator is placed by a construction robot, it doesn't contain any code.

I really couldn't reproduce this. Even your blueprint string correctly contains the code.
It even work with scripted robots that just script-revive combinators (as long as they have raise_revive = true in their reviving call, but if they don't, the error would actually be that the game crashes when you open the combinator)

Thanks for looking at this, I'll see if I can debug later.

I identified having Rampant loaded as a minimal precondition to replicate.

Here's a minimal test case:

http://s000.tinyupload.com/index.php?file_id=07772083266656193606 test.zip -- savefile without Rampant
http://s000.tinyupload.com/index.php?file_id=97180563321537517581 test2.zip -- savefile with Rampant

These should be otherwise mostly identical. However, when Rampant is loaded, the entity-ghost with the luacom_blueprint_data simply fails to appear in any find_entities_filtered.