Hi Tom,
Thank you for your interest in Syncfusion products.
I am afraid that I was unable to reproduce the mention issue while using ResetCustomization in MainFrameBarManger. We have tested the issue with simple sample and attached in following link.
http://www.syncfusion.com/uploads/redirect.aspx?&team=support&file=MenuItemsInsertTest-947496804.zipPlease have look at the above sample and let me know if you still face the issue. Could you please try to reproduce the issue in above sample that would be helpful for us to provide a better solution?
Regards,
Lingaraj S.