I got a
solution in the Playtime thread. It doesn't work when a 'Piano Roll Timebase' (view menu) is set to 'source beats'. Changing 'Piano Roll Timebase' to any other option fixes this issue.
As this seems to be an intended behavior when editor is in 'source beats' mode, it would be really helpful if a tooltip message would explain why it is not possible to change visibility, when hovering over the eye or lock icons.