Resolve Authentication Glitch in FastAPI App

I have an existing FastAPI application with JWT-based authentication, and the authentication flow is not working correctly.
The application is already developed. I need someone to identify the root cause, fix the authentication flow, and make sure the existing protected endpoints continue working correctly.

Current issues
• Login sometimes returns an authentication/authorization error
• JWT tokens are generated but aren't consistently accepted by protected endpoints
• Some requests return 401 Unauthorized even with what appears to be a valid token
• Token validation/dependency logic may be involved
• Existing users and database data must remain intact

Scope
• Review the existing FastAPI authentication implementation
• Reproduce the issue
• Identify the underlying cause
• Fix the JWT authentication/validation flow
• Test login and protected endpoints
• Check for related authentication issues
• Explain the cause and changes made

The project should be completed within 5 days.
The budget is $150 fixed price.
I already have the existing codebase and can provide repository access and setup information to the selected freelancer.
Please focus on fixing the existing implementation rather than unnecessarily rebuilding the authentication system.