Yeah, there was a change in how Dorico handles these between D4 and D5. I think I figured it out and detailed it in this post.
Here’s a new doricolib file for D5 that creates both a 0-Line and 1-Line instrument in the Custom category. Additionally, it creates a Clef for the 1-Line Staff that defines the line to be middle C for ease of input. The image appears wrong in the right panel, but is correct in the score. If you delete that old doricolib file and use this one instead, it should work correctly.
0and1LineInstruments-D5.zip (1.3 KB)