NOTE: it is expected to be fixed in the next Update of ARCHICAD 20Issue In case of exporting Favorites into XML or PRF format custom properties from the Categories and Properties tab page will not be transported or incorrect property values will get into another property.
Favorite export refers to the properties by an ID and not with their name. In case the project you want to import the favorites in does not contain that additional property ID, the property and its value will not be transported. If the project contains another property with the given ID the values will be mixed up.
Now it is only possible to create the property by hand in the new file and add it to the favorite manually.