Replaced EMS layouts
One thing you could try is changing the focus once you set the font size to use ems by hitting the tab key or clicking the apply button for the preview since DW does not save if the focus is not changed.
Perhaps you misunderstood, Dani. But when opening an EMS preset, the default text settings were not specified in EMS. Yes, I could reset each font setting manually in EMS, but that should not be necessary!
I ended up using the Preset Manager to remove all my existing EMS layouts. I then reimported a .Zip folder containing all the current EMS layouts.
Once I did that, the layouts worked correctly (as they did in the past).