It appears as though the value for the 'style select' toolbar option has changed in the TinyMCE json configuration from 'styleselect' to 'styles'. This doesn't appear to be mapping when using usync to import these files. We're using the latest version of Umbraco 13 (13.0.2 at time of writing) and uSync (13.0.1).
Saving down the property serialised the usync file back down to the new value (which is how I identified the change) but it's not clear this is required - might be a required change to usync possible to convert this value when importing (or at least display an error if it's present)