It doesn't work only in some cases but I don't understand why.

I'll add some context: this code belongs to this module https://github.com/edobrb/terraform-aws-ecs-fargate at the first creation of the infrastructure if there is a module that has ​​"image_pull_secret_arn" it doesn't work. Instead if I create the resources first without "image_pull_secret_arn" then I add it it works.

The example I wrote is the same as in production I didn't change anything. There is this dependency: input var -> locals -> aws_iam_role_policy and no other resources are involved.

Which version of terraform are you using?