Using Ulysses with Marked
In Ulysses, dragging a file from the sheet list to Marked 2 will only work for files within "External Folders." Internal Ulysses documents are not stored on disk in a way that Marked 2 can read, and the preview only shows up initially because when dragging out Ulysses creates a temporary preview that is overwritten after changes.
To preview Ulysses sheets in Marked, use the Quick Preview (⌘6).
1. Press ⌘6 to open the Quick Preview palette
2. Select "Text" as the preview type from the dropdown at the top
3. Set the format to "Textbundle"
4. Use the dropdown on the right side of the button at the bottom to choose Marked 2
Once these are set, typing ⌘6 and pressing Return will open/update the preview in Marked. This will need to be done manually any time you want to update the preview in Marked 2.