I wonder if this is just a Windows version of Dorico issue? When I added woodwinds, brass, and percussion, the ‘Add Ensemble’ feature had no problem adding a bracket… but, when I typed ‘string ensemble’ in the Shift-E text box, it adds the instruments but with no bracket…
It’s easy enough to fix in the Engrave mode, but I don’t want to add the bracket that way… if the add ensemble should do it. I also tried entering the instrument names with commas and it didn’t add the bracket that way either (for strings only).
I would suggest deleting the bracket change you’ve created in Engrave mode (by way of its signpost) and then checking the settings on the Brackets and Braces page of Layout Options.