It’s likely that the end-repeat below in the Vc. 3 has something to do with it. I’d expect Dorico to be able to put that end repeat close but not directly underneath the open repeat.
There are still other instruments off both ends of the page, so I really can’t be sure what the cause is. You can probably work around it using the Note Spacing tool in the left panel of Engrave mode.
This is a situation where it’s much easier if you post a project, not a picture.