In this configuration, it appears that the value for module.iam_policy_mod1_backup1.arn inside the concat argument is coming from the managed resource module.iam_policy_mod1_backup1.aws_iam_policy.policy.
This means that during planning the module.iam_policy_mod1_backup1.arn value will be unknown. Because the value is unknown, the result of the concat function will be unknown, since it cannot be determined until apply time how many non-empty values there will be in the list.
Using the list directly works, because the number of items is known, even if they were to end up as null values.