What was going on
After upgrading the IBM Cloud Terraform provider to v2.5.0, ibm_iam_user_invite was no longer idempotent: if a user was already an active member of the account, terraform apply failed with: [ERROR] User invite unsuccessful: user(s) [...] are already ACTIVE in account ..., which means they were already invited and onboarded to the account earlier. Previously the invite was a no-op. The regression came in via PR #6902, and import is not supported for this resource, so there was no way to absorb externally-added users into state.
What fixed it
This was a provider regression in v2.5.0 , upgrade to v2.6.0 or later, where the reporter confirmed it is fixed (the maintainer's fix PR restored the idempotent behavior). If you are stuck on v2.5.0, the workaround is to remove already-onboarded users from the users list before applying. Source: https://github.com/IBM-Cloud/terraform-provider-ibm/issues/6953