It turns out that the cases in which this script loses the font/size applied to text runs are rooted in a fairly deep and architectural bug in the interface between Applescript and OmniGraffle.

In a number of respects (principally involving texts) the state of a document seen by Applescript differs from that seen by the GUI. Sometimes different levels of zoom show different snapshots of the editing history of the document.

In this case, to reconcile the state of play seen by Applescript with that seen by the GUI, you would need to save, close, and reopen the document before you run the script ...

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