Dear Mike,
It happened on the "Add/Edit Custom Entry" screen. I have a lot of custom user dict entries, here's what I usually do to edit one of them:
1. Select the existing entry
2. Select from the menu: "Modules" , "Add/Edit Custom Entry..." or simply press the shortcut "Menu - a"
3. Now the "Add/Edit Custom Entry " screen appears with my existing custom definition :
Simp: xxx
Trad: xxx
Pron: xxx
Defn: xxx
4. Tap screen anywhere on the "Defn" field area, then the "Defn" field would be editable with no warning, but not the other field (Simp, Trad, Pron). The changes definitely appear in the database.
You're right Mike, only the Defn field is editable without unlock warning. On second thought I realize perhaps it was designed that way, not a bug at all. It allows quick change to definition only. I like it that way.