Hi Julie,
Thank you for your interest in Syncfusion Products.
I have tested the above mentioned issue with the simple sample. I could see the issue is reproduced with our 5.2 version, it has been internally fixed with our 6.2 and further release. Please refer to the below screen shot for your reference.
http://websamples.syncfusion.com/samples/Tools.Windows/F77481/ShowMnemonic_BarItem.zipPlease let me know if any concerns.
Regards,
Hema