When trying to use the request body stream as content for another HttpRequest, the dotnet-isolated function app seems to have issues with certain backend servers which see the request body as empty. The exact same http call in a in-process .NET 8 function works. Materializing the request body and using StringContent for the HttpRequest Content property eliminates this issue.

in-process:

using System.IO;
using System.Threading.Tasks;
using Microsoft.AspNetCore.Mvc;
using Microsoft.Azure.WebJobs;
using Microsoft.Azure.WebJobs.Extensions.Http;
using Microsoft.AspNetCore.Http;
using Microsoft.Extensions.Logging;
using Newtonsoft.Json;
using System.Net.Http;
using System.Net;
using System.Threading;

namespace FunctionApp2
{
    public static class Function1
    {
        [FunctionName("Function1")]
        public static async Task<IActionResult> GeneratePdfAsync(
        [HttpTrigger(AuthorizationLevel.Function, "post", Route = "documents/{tenant}/{engine}/{template}")] HttpRequest body,
        string engine,
        string tenant,
        string template,
        CancellationToken ct)
        {
            using var client = new HttpClient() { BaseAddress = new Uri("https://my-backend-func.azurewebsites.net/api/") };
            using var request = new HttpRequestMessage(HttpMethod.Post, new Uri($"pdf/{Uri.EscapeDataString(tenant)}/{Uri.EscapeDataString(template)}", UriKind.Relative));
            //request.Content = new StringContent(await (new StreamReader(pdfRequest.Payload)).ReadToEndAsync());

            request.Content = new StreamContent(body.Body);
            using var response = await client.SendAsync(request, HttpCompletionOption.ResponseHeadersRead, ct).ConfigureAwait(false);
            if (response.StatusCode == HttpStatusCode.OK)
                return new FileStreamResult(await response.Content.ReadAsStreamAsync(ct), "application/pdf");
            else
                return new BadRequestObjectResult(await response.Content.ReadAsStringAsync());
        }
    }
}

dotnet-isolated:

using System;
using System.Threading.Tasks;
using Microsoft.AspNetCore.Mvc;
using Microsoft.AspNetCore.Http;
using System.Threading;
using System.Net.Http;
using System.Net;
using Microsoft.Azure.Functions.Worker;
using System.IO;
using Microsoft.Azure.Functions.Worker.Http;

namespace FunctionApp1
{
    public class Function1
    {
        [Function("Function1")]
        public async Task<IActionResult> GeneratePdfAsync(
        [HttpTrigger(AuthorizationLevel.Function, "post", Route = "documents/{tenant}/{engine}/{template}")] HttpRequestData req,
        string engine,
        string tenant,
        string template,
        CancellationToken ct)
        {
            using var client = new HttpClient() { BaseAddress = new Uri("https://my-backend-func.azurewebsites.net/api/") };
            using var request = new HttpRequestMessage(HttpMethod.Post, new Uri($"pdf/{Uri.EscapeDataString(tenant)}/{Uri.EscapeDataString(template)}", UriKind.Relative));
            //request.Content = new StringContent(await (new StreamReader(pdfRequest.Payload)).ReadToEndAsync());
            request.Content = new StreamContent(req.Body);
            using var response = await client.SendAsync(request, ct);
            if (response.StatusCode == HttpStatusCode.OK)
                return new FileStreamResult(await response.Content.ReadAsStreamAsync(ct), "application/pdf");
            else
                return new BadRequestObjectResult(await response.Content.ReadAsStringAsync());
        }
    }
}

I expected the behaviour of this simple POC to match between the different host runtimes. I have not found anything online explaining this behaviour.