Can't Override Block Title

I'm using Fusion Starter with minimal local css and maybe one custom template. Block titles override has been working fine until recently. Lately when I try to override a block title, the override is not saved. Yesterday the problem seemed to resolve itself after a few tries, but no luck today. Does anybody have an idea how to troubleshoot this?

Yesterday the problem occurred with a custom menu block (populated from views). Today it is an exposed filter block from views.