Assuming you are using FMX, it is an issue in using a form with disabled border. We have tested this here and this issue is also reproducible with a default TForm in FMX, with a BorderStyle property of None. As this is an underlying FMX framework issue, there is little that we can do on our side to fix the issue. Please report this issue to Embarcadero.
Unfortunately even in VCL this is behaving differently with a borderless window. We'll investigate what exactly is going wrong with this. To fix this issue, you need to leave out the TTMSFNCRibbonForm and inherit from TForm again.
On what operating system are you using this?
Windows 10 Pro 1903
We'll investigate what exactly is going wrong and see if there is a workaround, what I was trying to explain is that the issue is reproducible without our components, when using a borderless window. So the issue is only partially due to the TTMSFNCRibbon. It's also technically challenging to fix these kind of issues. We have added this on our todolist.