Maybe at some point the number I has become an explicit part of the instrument’s name. When you click the three dots to the right you get a contextual menu with an option to change the name (or reset it to default). If the explicit ‘I’ isn’t there at all, then I guess it’s a bug, and I’m sure the developers might want to see the project itself to examine it. In that case you might try the following: create a new empty section player, drag the soprano (the blue instrument) to it (this moves all the music along with it), and delete the original soprano, which now is probably called Empty-handed player. Make backups first before an operation like this.
What happens when you first change the name to something else, say ‘Sopranooo’, apply the change, then change it back? Maybe you break the remnant of the automatic numbering with it? Or even more thorough: assign a totally different instrument (say flute), then change it back to soprano?
There are no known bugs in this area in Dorico. If you find that the instrument name continues to show a number when only one instrument of that kind remains in the project, chances are you have overridden its name, and need to reset it. If you want us to look into it further, please provide the project file.