3f7323cbb regenerates its Param for each SubPlan by traversing the targetlist. But ignore one point: initplan is not in targetlist. This will result in a failed assertion, or an error like "unexpected PARAM_MULTIEXPR ID: 131074"
Since initplan SubPlans do not have args lists, I think it's OK for them to share output parameters. So maybe we do not need to do the SS_make_multiexprs_unique trick for initplan SubPlans?