Unable to assign Contributor role in Azure subscriptions (RBAC appears restricted despite Owner role)

Summary / Problem description

I am experiencing unexpected RBAC behavior across multiple Azure subscriptions in the same tenant.

Although I am Owner on the subscriptions, I am unable to assign the built-in Contributor role at subscription or resource-group scope. The role exists and is visible when listing all roles, but it does not appear as assignable during role assignment (only Reader is available).

This behavior is consistent across:

DEV

TEST

PROD subscriptions