If the user set in Editing Options (accessed from standard Editor toolbar) in Attributes tab that he/she wants to "Display the attribute dialog before storing new features" ArcMap displays standard ArcMap attribute window in a modal dialog.
If target feature class has a class extension registered and implemented IObjectInspector control the expected bahaviour is a customized attributes dialog, not a standard one. It's inconsistent as user sees customized attributes page for existing features but a standard one for new features.
See attached picture: 2 different attributes windows are shown at the same time: point with OBJECTID=2 is shown using Attributes toolbar button (modeless customized dialog). point with OBJECTID=3 is popped up after user just created a new point using Sketch tool.
PS. ESRI support suggested to implement additional interface IExtendedObjectInspector. This worked to solve the problem. I love that ArcObjects wood of interfaces 🙂