Still not entirely sure why it happened here, but this solved it:

externalApi
    .WithHttpsEndpoint()
    .WithExternalHttpEndpoints()
    .AddEnvironmentVariable("ASPNETCORE_URLS", "http://0.0.0.0:8080")
    .AddEnvironmentVariable("DOTNET_URLS", "http://0.0.0.0:8080")
    .AddEnvironmentVariable("ASPNETCORE_FORWARDEDHEADERS_ENABLED", "true");