Welcome to the ODK forum, @Etienne_Le_Rossignol! We're glad you're here. When you get a chance, please introduce yourself on this forum thread. I'd also encourage you to add a real picture as your avatar because it helps build community!
The only group names we remove from filenames are those belonging to non-repeat groups, so what you're reporting would be a bug. In any case, v1.11 is a very old version. Could you run Briefcase v1.17 and report back the names you get with that version? You can get it here: ODK Briefcase v1.17
Also, it would help us to have the form you're using to try to reproduce the issue. Could you attach it here, please?