I now have build 1677, and the problem persists. However, I've learned a little bit more: first, it turns out that the buttons ARE there, they're just invisible because they're black-on-black! If I click on the right side of the black bar, I can edit the event; clicking in the middle makes it ask if I want to delete it. (And while I'm pushing on the button, the button turns dark gray instead of black, and I can briefly see the hidden black text.)
I should also note that I'm using CalenGoo's "Android 3/4" design. If I switch it to 2 or 5, the buttons are visible. So it might not be related to the Edge-to-Edge thing?