Removing the [] in the parameter and in the expression declaration has no effect, the parameter is still not displayed when the expression is set from the expression list.
Anyway, the [] are set in informative strings (set to be displayed for the user not used for the functionning like the naming "getBoard" is) and shouldn't affect the functionning of the behavior, aren't they ?
This is not blocking, so it can wait.
Users will have to be a little careful using the behavior until I figure a way to correct this.
Thank you for your answer and sorry to bother you during your "time off".