Custom fields with names under 3 characters can't be picked as To/CC issue fields for custom emails

To reproduce:

• Create a custom field (single-/multi-user or short text) with a 1- or 2-character name (e.g. "Cc")
• Go to Script Listeners > Send a custom email
• Type the name of the field into the 'To issue fields' or 'CC issue fields' box

Expected: The field appears in the dynamic drop-down and can be selected.

Actual: No fields appear and the drop-down prompts you to 'Add 1 more character...'