In lightweight the issue is not observed:
<telerik:RadRibbonBar runat="server" RenderMode="Classic">
<Tabs>
<telerik:RibbonBarTab Text="home">
<%-- duplicate the group and items inside to replicate more easily --%>
<telerik:RibbonBarGroup Text="Documents">
<Items>
<telerik:RibbonBarButton Text="Item 1" />
</Items>
</telerik:RibbonBarGroup>
</telerik:RibbonBarTab>
</Tabs>
</telerik:RadRibbonBar>
Thank you for reporting this Daniel,
As a temporary workaround you can use the following style
<style>
.RadRibbonBar .rrbButtonGroup.rrbDetachedGroup.rrbHidden,
.RadRibbonBar .rrbButtonGroup.rrbDetachedGroup.rrbHiddenGroup {
display: block !important
}
</style>
Regards,
Peter Milchev
Progress Telerik