amp-ad tag injected into node body gets stripped out

Problem/Motivation

I have created a custom filter which extends FilterBase and injects an <amp-ad width=300 height=250 type="doubleclick" data-slot="xyz"></amp-ad> after the second paragraph of an article node body.
However this tag gets stripped out. I suspect this is done by the AMP Text Formatter.
This does not seem to be related to https://www.drupal.org/project/amp/issues/2871805 since my amp library version is 1.1.4

Steps to reproduce

Proposed resolution

Remaining tasks

User interface changes

API changes

Data model changes