One last post - the error was caused by me, hence no change to the templates is needed, at all! 
I’m placing my API requests using the requests python library, and I was submitting the payload using parameter params=payload rather than data=payload, hence my oversized payload was handled in the “wrong” way.
I changed my call from this
r = requests.put(apiWebsite + '/posts/' + str(apiPost), params=payload)
to this
r = requests.put(apiWebsite + '/posts/' + str(apiPost), data=payload)
Sorry! 