When the Groovy script uses a Map as return value, the default value of the parameter is invalid. It uses the Map values (descriptions) instead of the Map keys (actual parameter values).

Example :

return ['A': 'Description for A:selected', 'B': 'Description for B', 'C': 'Description for C:selected']

If the job is triggered by a timer, the parameter value should be :

A,C

but the actual parameter value is :

Description for A,Description for C.