iam.Role.fromRoleArn() with { mutable: false } creates role policies in 1.21.1. This did not occur in <=1.20.0

Reproduction Steps

1. Clone my reproduction repo: https://github.com/charlocharlie/mutable-role-repro
2. npm i
3. cdk synth
4. Observe template
5. Downgrade to 1.20.0
6. npm i
7. cdk synth
8. Observe template without any role policies

Or just view the synth-1.20.0.json and synth-1.21.1.json in the repro.

Error Log

Causes many issues when deploying to a role-strict environment.

Environment

• CLI Version : 1.21.1
• Framework Version: 1.21.1
• OS : macOS
• Language : Typescript

This is 🐛 Bug Report