When I activate the “Instrument Transposition” in my score, there is a gap between the letter and the flat symbol :
Hi Olivier and welcome to the forum!
That probably would be a kerning problem in the font you’re using, or even more probably a too large space bearing before the flat sign. [in Academico, this side bearing is set to 40, I just checked, as per the picture below] Depending on the font, you might be allowed to modify this with a font tool (I have Glyphs, and this is a very simple operation). But you might as well not be allowed to touch this, and then the easy workaround would be to change that font to another that does not exhibit the problem.
Can you check using Academico or Nepomuk (side bearing 60)?
Note that if it’s allowed to be modified, I would gladly do that for you, it’s a two seconds operation.
Hi Marc, thank you for the answers. You are right! In the Character Styles dialog, I do choose the same font as I use for other texts in my score. When a go back to Academico font, it’s ok. Thank you 
What font were you using?
You can try to switch to Times New Roman in Character Styles dialog, you should be able to reproduce the problem.
PS : I’m quite new to Dorico, old Finale user Do I have the right to speak French here ? 
Actually this is an international forum, the etiquette is to write in English so that everyone can understand and share. If you prefer writing in French, don’t forget to add an automatic translation of your text in English. And if you really want to write in French, you’re welcome in the facebook group: groupe des utilisateurs francophones de Dorico
I’ve opened Times New Roman in Glyphs, and there’s no flat sign in there, so I’m wondering where it’s taking it from (probably a fallback font, and I admit I know close to nothing about that). But from what I see, that fallback font has extra large side bearings!
I’ve been digging that thing a little bit more, and the exact same problem appears on the MuseScore forum (from 2019), especially with Times New Roman (because it does not have that flat glyph).
