If two selected photos have different entries in a particular field, LrC displays <mixed> in that field. In version 9.4 (and earlier), after synchronizing metadata, LrC would update the display of the field with previously different data and display the field showing the now identical data. In LrC 10.0.0, the field is not updated to display the data. It still shows <mixed>. After deselecting the selected images and then reselecting the images, the field correctly displays the updated (identical) contents.
Steps to reproduce problem:
In the Library, select two images for which a given metadata field (e.g., Title) are DIFFERENT. In the Metadata panel, the field will display <mixed>
Click the Sync Metadata button at the bottom of the right side panel. Be sure that the field in question (e.g., Title) has its checkbox checked in the Synchronize Metadata dialog box that appears.
After clicking the Synchronize button, the field (e.g., Title) will still show <mixed>, rather that update itself to the (now identical) contents of the field.