"Undo" loses data

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.

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