I tried the following and the custom context menu does not apply:
<telerik:RadEntry x:Name="telerikEntry">
<FlyoutBase.ContextFlyout>
<MenuFlyout>
<MenuFlyoutItem Text="Custom flyout telrik radentry"
Clicked="MenuFlyoutItem_Clicked">
</MenuFlyoutItem>
</MenuFlyout>
</FlyoutBase.ContextFlyout>
</telerik:RadEntry>
It works for MAUI Entry. Provide this menu for Telerik MAUI RadEntry too.