OCI Always Free instances SSH port 22 connects then closes before banner/key exchange

Summary:
I need help with an OCI Always Free Compute SSH access issue. Multiple Ubuntu instances in us-phoenix-1 show the same behavior: TCP port 22 is reachable, but the remote side closes the connection before the SSH banner/key exchange and before any key authentication.

Content:
Environment:

• Region: us-phoenix-1 (US West / Phoenix)
• Account/tenancy name: t14892335
• Compute: OCI Always Free / Ubuntu instances
• Current instance: codex-clean-0601-1428, VM.Standard.E2.1.Micro
• Public IP examples: 132.226.80.****, 161.153.32.****, 137.131.50.****
• Existing Oracle incident reference: 260603-001025

Observed error:
ssh -vv -i <private-key> -o IdentitiesOnly=yes -o StrictHostKeyChecking=no -o ConnectTimeout=15 ubuntu@<public-ip> hostname

Connection established.
Local version string SSH-2.0-OpenSSH_10.2