I’m not a heavy user of Impress, but I have never experienced the “custom animation lockout” that you have.
It might be worth looking through the relevation section of the Impress Guide: ch. 9, “Slides Shows: Transitions, animations, and more” - around p. 13, of the v.4.0 version. There’s a bit that walks through adding/altering animations. It might be something else entirely, of course, but that would be a good place to start.
If that doesn’t crack it, then it would be worth adding a bit more info: what operating system are you on? confirm the version of LibO? what format are you saving in? etc.