HTTP Error 411. The request must be chunked or have a content length

Clients
1

SS 5.10.3

I upgraded the project from .net core 2.2 to 5.0 and I have a flutter app that sends some simple posts with just an int id {id:100} and they are all failing now with: HTTP Error 411. The request must be chunked or have a content length.

All the GET’s work completely fine.

This is what is returned with the POST:

<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01//EN""http://www.w3.org/TR/html4/strict.dtd">
<HTML><HEAD><TITLE>Length Required</TITLE>
<META HTTP-EQUIV="Content-Type" Content="text/html; charset=us-ascii"></HEAD>
<BODY><h2>Length Required</h2>
<hr><p>HTTP Error 411. The request must be chunked or have a content length.</p>
</BODY></HTML>

I understand the transfer encoding is set at the client level but I would have assumed the service stack client would be setting it?