You should be able to sync Character properties (eg. Nickname) to Scrivener where they will appear as Custom Metadata. The only issue with properties like these is that every document in Scrivener will have to contain the custom metadata property, even if it is not relevant (eg. your documents in your manuscript would also contain a custom metadata type called “Nickname”)
Are these options set up to sync on your current project? Open Sync Settings (the cog on the Sync binder), click Show Options and navigate to the “Show Properties” menu. What is shown in the dropdown next to those properties on these page? Do they say “None” or something else? If they say None, then you need to choose an option from the dropdown to list for what you want to sync to. If they don’t say None, and then the data that you are trying to sync isn’t coming through to this location in Scrivener, then there may be a bug. If you could email your timeline and Scrivener project to email@example.com, then we can look into it further.