Hi, I seem to remember that this problem has been discussed a number of times before.
As Trevor points out, the \partcombine code creates its own contexts and adds the combined notes to them depending on whether or not they are combined. I think (from memory) it creates 3 contexts, "one", "two" and "both" which it combines visually such that you cannot tell.
Whichever context you pick to attach your lyrics, it will have holes: "one" will not contain combined notes and "both" will not contain any events where notes are not combined. It is a nuisance, but \partcombine is a bit of a hack anyway.
There are a number of solutions I think to the problem, one of which is the DevNull solution which is not ideal in itself, where you create another context into which one of the melody lines is put and attach lyrics to that. The DevNull context actually doesn't put anything out so it is an invisible context - it just provides the musical events onto which you
can hang the lyrics.
I would search LSR and the archives for "partcombine" and look for the previous discussions - last one around 2 months ago I believe!