If I select a leader and try and edit the text in the property window, I can type one character before the caret moves to the beginning of the line. Initial placement of the caret seems a little funny as well (it doesn’t often end up where I click on the first try). Double clicking to edit leader text seems to work fine.
Looks like I missed the spot to drag to resize the leader edit text area… We’ll need to make that more discoverable but in the meantime, try hovering right over the B I U options to see the vertical arrow.