Sorry, this is embarrassing but it seems iex no longer opens an iex window. I was using it in 2016 without problems.
For example, iex > 'Open iex session in bottom pane' does nothing except shows symbols for alt-cmd-l-down at the bottom of Atom but that key combination doesn't do anything.
I'm sure I'm doing something wrong!

As it happens, alt-cmd-l-down does the same as alt-cmd-down:

Any ideas?