Hi,
In my opinion your customer-exit variable should show multiple lines in internal table i_t_var_range. It is input-ready and configured for multiple single values, the user is able to enter multiple single values so it should be reflected this way in internal table i_t_var_range at i_step = 2.
I suggest to raise an SAP Support message for it. It could be a program error in standard SAP.
Best regards,
Sander
PS: I agree with Tom to consider a real Analysis Authorization approach with an input-ready authorization variable.