just wanted to add that i've had this behaviour of the buttons not just since the 2018 release. We're now on 2018 too but this has been happening to the buttons at least since 2012 or before. It's unclear to me when they disapear exactly, but they can be there multiple sessions and then they're gone without having other issues with inventor. It also happens in every environment; assembly, part, and drawing too.
i would greatly appreciate a solution for this as the icons also help me identify easily if i need to check something in or out.
regards,
Misha