I have the same problem, my labels are all lost and its not allowing me to label using the ESRI std engine but if i check the maplex option it works.
I found the following in the Prohelp ."When you import a map document, you will always get at least one map and one layout, even if you never made any edits in layout view in ArcMap. Each data frame is imported as a separate map. All map layers will be imported, along with their symbology and other layer properties. Symbols are imported, but the .style files used to create your maps in ArcMap will need to be converted separately to the .stylx format used by ArcGIS Pro. The Project pane provides a detailed log of what was imported."