IAM Assignment Behaviours

I’m currently working on an infrastructure on GCP, and due to the complexity, i had to segment the code in multi-part representing parts of this one.
Lately, i have been struggling with some IAM bindings due to the mechanism behind the terraform resource.
To keep each part with the maximum segregation, i had to split some role assignment’s inside each autonomous part.
But when i’m trying to assign the same role with a new user each time, all the previous assignment are destroyed. This mechanism is still very curious for my point of view.

I have worked with other cloud provider and never this behaviour has happened. Have you encounter any similarities in your script, and if yes have, you resolved it and how ?