Why do I need to recreate a brace or bracket for a player that is already avaliable on the Setup Page?
If I select a two-stave player to be included in the current flow, it appears without connecting barlines or brace.
Is there a general preference for this in the Engrave Options that I have missed?
This was not an issue pre-v4.
The problem will be that you already have a bracket and barline change at that position or an earlier one. The way that those changes work is that they store the explicit brackets and braces (and barline joins) for every staff in your ensemble. So when you subsequently add more players that introduce new staves, those won’t have any brackets or braces in your existing change item, and therefore won’t show any.
You can either delete your existing bracket and brace change and recreate it, or manually add brackets/braces as needed to the new staves.