I'm having the same issue with the missing scripts. The files exists but they're not set correctly for some reason.
For example the Pun Demo Manager script in PunGame is not set to PunDemoManager.cs.
I'm not sure if this is useful info but I checked the guid before (with it saying missing) and after manually changing it to the correct file.
Before:
m_Script: {fileID: 11500000, guid: a11ce138cfa7b57459bbbe2c3ffec0b7, type: 3}
After:
m_Script: {fileID: 11500000, guid: 8345c0e8756943b4389f838450519913, type: 3}
PunAgent.cs.meta, PunDemoManager.cs.meta and TrafficMonitor.cs.meta have file modified dates of 24-8-2025 and all of those have a mismatched guid compared to what's in Demo.unity.
I'm using Unity 6000.2.14f1 with the UltimateCharacterController and PUN addon downloaded trough the Opsive website on 30-11-2025.
Hopefully this issue gets resolved soon.