Purging output cache in .NET minimal API has no effect

Question

In a .NET 8 minimal API I've implement output-caching on one endpoint (in a Carter module).

const int cacheDurationInMinutes = 10;

var clientGroup = app.MapGroup("/api/v{version:apiVersion}/client")
                     .WithApiVersionSet(_apiVersionSet.Value)
                     .WithTags("Client");

clientGroup.MapGet("/", GetClient)
           .WithName(nameof(GetClient))
           .WithOpenApi(operation =>
                        {
                            operation.Summary = "Obtains the current configured client.";
                            return operation;
                        })
           .CacheOutput(policyBuilder => policyBuilder.Expire(TimeSpan.FromMinutes(cacheDurationInMinutes))
                                                      .Tag("tag-client"));

You can see here that I've set the cache duration to be 10 minutes, and I've declare the tag tag-client on the cache policy.

If I add a breakpoint at the start of this method, then debug from a .http file, it hits the breakpoint on the first call. If I send another request it doesn't hit the breakpoint - good, caching is being implemented.

In Program.cs I have...

app.MapPost("/purge/{tag}", async (IOutputCacheStore cache, string tag) =>
                            {
                                await cache.EvictByTagAsync(tag, default);
                            });

I then send a call to /purge/tag-client: it hits that ^^ anonymous function and then I receive a 200 (OK) response.

If I then send another request to the method which implements caching I still doesn't hit the breakpoint - presumably because there's a cached result?

Why is there still a cached result after I've purged it?