**Might** be related to bug 1576646, but not sure. Microsoft's for Office uses (used?) a lot of `<span>` elements to manage undo transactions by themselves. It might cause this bug if Outlook's editor does similar things or is same editor.
Bug 1755342 Comment 4 Edit History
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**Might** be related to bug 1576646, but not sure. Microsoft's editor for Office uses (used?) a lot of `<span>` elements to manage undo transactions by themselves. It might cause this bug if Outlook's editor does similar things or is same editor.