Either I've had a massive brain freeze (entirely possible), or I've found a serious bug in 2.7. Whole blocks of cues are disappearing. It seems to be related to "Undo".
The first time it happened, I assumed it was PEBKAC, but it happened 3 or 4 more times. I know the cues were written, because I was able to restore them from previously-saved show files. I don't know if other data are erased.