Presence of <SVG in POSTed content results in 403

I've created a basic PHP Micropub client to talk to my WordPress Micropub endpoint. It uses form-urlencoded and calls the endpoint with file_get_contents()
This works all as intended.

When I include an SVG icon in the html content however the endpoint returns a 403 error. It was pointed out to me that this might be an issue with WP core, not with the plugin ( https://core.trac.wordpress.org/ticket/30377 https://core.trac.wordpress.org/ticket/54244 ). However that seems to be about GET requests and urls, not POST requests.

Original HTML that results in 403:
'` Some text here`'

but pairing it down showed that

'`some text more text`'

also gets a 403 and even

'`some text

Start by reproducing the POST to the WordPress Micropub endpoint with the minimal `

Written by the indexing model from the issue text.