As always, it is a bit of a guess without seeing the actual project file. But what I think is happening here is a combination of factors:
- Pitched Percussion and Unpitched Percussion are considered separate instrument families by Dorico, each with a separate checkbox under Layout Options > Staves and Systems > System-attached Items.
- By default, only instruments from the same family are bracketed together.
- System-attached items will only appear above the first bracket for a given instrument family, i.e. no bracket → no rehearsal mark.
There are a few ways around this. First, a trivial one: this looks like a regular symphonic score, where the almost-universal standard is to have system objects in two places only: at the top and above the strings. A conductor will not be looking for them above the brass or percussion, and to put them there kind of looks like a misprint.
That said, you could add a manual bracket around the whole percussion section. Temporarily turn off both Condensing and Instrument Changes for the score layout, then in Engrave mode, on page 1 select something in the topmost and bottom-most percussion instrument and click the bracket in the left panel.
Alternatively, in Dorico 6 you can also choose Show system-attached items above: Specific Players and manually click six checkboxes each for Flute 1, Horn 1, Violin 1 and whatever’s the topmost percussion instrument.