Decompress JWT payload in ASP.NET Core

Question

I am setting up an ASP.NET Core Web API to use JWT Bearer authorization. It's built with .NET 5 and uses the Nuget package Microsoft.AspNetCore.Authentication.JwtBearer.

The code in Startup.cs looks like this

services.AddAuthentication(JwtBearerDefaults.AuthenticationScheme)
  .AddJwtBearer(JwtBearerDefaults.AuthenticationScheme, options =>
  {
    options.Authority = "...";
    options.Audience = "...";
    options.Events = new JwtBearerEvents
    {
      OnTokenValidated = (context) =>
      {
        return Task.CompletedTask;
      },
      OnAuthenticationFailed = (context) =>
      {
        return Task.CompletedTask;
      },
    };
  });

The OnAuthenticationFailed event occurs, and the context has an Exception property with the Message:

IDX12723: Unable to decode the payload 'System.String' as Base64Url encoded string. jwtEncodedString: ''.

That Exception has an InnerException property with the Message:

Unexpected character encountered while parsing value: U. Path '', line 0, position 0.

When I paste my JWT token into https://jwt.io/ I can see HEADER:

{
  "typ": "JWT",
  "zip": "DEF",
  "kid": "b/O6OvVv1+y+WgrH5Ui9WTioLt0=",
  "alg": "RS256"
}

PAYLOAD:

UQ[...very long binary looking string]

I find it suspicious that the InnerException message mentioned 'U', which is the first letter of my deflate compressed payload. It looks like the payload isn't being decompressed.

I wrote a small (separate) program to split the JWT between '.'s and then used a DeflateStream to retrieve the contents of the token's payload, which are - as I expected - a set of claims in valid JSON encoded in UTF-8.

Is Microsoft.AspNetCore.Authentication.JwtBearer unable to decompress token payloads? Is there an option I need to check to enable decompression? Or is there another library that will do this for me? Or can I even extend it myself, seeing as I am able to access the payload manually?