This is a bit of a mess:
https://github.com/gohugoio/hugo/issues/8910#issuecomment-903158600
Related docs:
https://gohugo.io/content-management/summaries/#summary-splitting-options
Regardless of which summary method you use, do not pass it through markdownify.
The easiest way for you to have markdown in summary not stripped of tags is to use manual summaries, by placing <!---more--> to identify where the summary should end.
---
title: This is a Blog Post
date: 2023-03-01T09:52:19-06:00
draft: false
---
# This is a heading.
Consectetur consequatur magnam vitae omnis magni tenetur, perferendis ut, animi! Sed quisquam dolor dolorum tempora eum, deserunt earum Iure maiores maiores facere iste aspernatur, sint ipsum vero Laboriosam molestias ducimus nulla veniam atque. Harum voluptate maiores earum omnis reiciendis illo.
<!--more-->