Before adding the "Checklists" custom field, make sure that no such field has already been created on your instance. Duplication of the "Checklists" field will lead to unpredictable add-on behavior. Smart Checklist will not recognize which field to use as a data storage.
In case 2 or more fields are added, users will see the following message:
Solution: remove all "Checklists" duplicates. Leave only 1 field.