I would have thought that clicking Undo would be enough, but it doesn't seem to work!

In a translation in Studio 2019, I merged 3 segments (having first enabled this in Project Settings), but then I realized this would mess up the formatting too much. So I would like to undo the merge - but the segments seem to have disappeared. The numbers in the left-hand column jump straight from 44 to 48, and the text of the source segments is nowhere to be found.

When I click undo (ctrl+Z), all my other actions can be undone, but the segment merge doesn't appear in the list of actions.

Any insights?

Thanks very much in advance!