The problem is fairly easy to recreate: Say, I have two staves (sopranos and altos, for example), and I have written a line for the upper staff first (say, two bars; i.e. b. 1-2) and then another line for the lower staff (another two bars, i.e. b. 3-4); finally, both staves have music (the following to bars, for example, for a total of six bars). This is the setting. Next, I write some lyrics on the lower staff first (b. 3-4), then I write some lyrics on the staff above (b. 1-2). Now, for the problem to happen, the last syllable of the lyrics on the upper staff needs to be a melisma. I now select the first two bars from the upper staff and copy them to the lower staff. Voilà! Although the music on bars 3-4 remain unaltered, the lyrics are erased.
I can see clearly why this is happening, but I don’t think it should behave this way.