App must be restarted for each subsequent snapshot. Not the same behavior as Ksnapshot. This can be tedious for multiple grabs.
In general you don't have to restart Spectacle to take multiple screenshots, you can simple click on "Take a New Screenshot". Perhaps you are using one of the global shortcuts instead of start Spectacle directly (e.g. by pressing the Print key, from the menu or with "spectacle" from a shell)?
No answer, closing as WORKSFORME. Perhaps the global shortcut (which saves directly) was used instead of opening Spectacle as an app.
This indeed seems to be working properly now after desktop upgrade.