You have a Microsoft 365 E5 subscription.

Microsoft 365 Copilot can only process (summarize, answer questions about) a labeled file when the user has the Copy and extract content (EXTRACT) usage right granted by the applied sensitivity label.

From the table of labels:

Label1 → VIEW only (no EXTRACT) → Copilot cannot summarize.

Label2 → VIEW and EXTRACT granted → Copilot can summarize.

Label3 → VIEW and EXPORT (no EXTRACT) → Copilot cannot summarize.

Since File2 is labeled Label2, it’s the only file for which User1 has the required EXTRACT permission, so it’s the only file Copilot can summarize.