The latest build 1.4.3 (584) removed the ability to edit Unicode value for character. I am using “Private” character space to use icons as characters. Without being able to assign custom character names in private space I can no longer add new icons.
Never mind, I’ve figured it out. It is not a bug, but a “feature”. To solve this, go to File > Font Info > Other Settings, then toggle ON “Don’t use nice names”. This will enable the field to be editable.