Hi Christian,
Are you showing any Child forms from the Main from? If so does your child forms have ChildFrameBarManager?
While switching between Child forms, a gap appears between the bars. This is an known issue.
Already a Bug Report has been created for this issue. Here is the link for the same.
Defect #2523 - Gap appears between menu Bars when switching child forms.This issue is under Testing and we will let you know, when it is solved.
Thanks for using Syncfusion Products.
Regards,
Purusothaman.R