Cannot display summary on index page

[mather]

Describe the bug
When you use "read more" (<!--more-->) in article, index page won't display summary (under title).

summary in front matter correctly works.

To Reproduce
Steps to reproduce the behavior:

1. Use "read more" (<!--more-->) in article.
2. Build.
3. See index page.

Expected behavior
The part above "read more" appears as a summary.

Screenshots
If applicable, add screenshots to help explain your problem.

Desktop (please complete the following information):

• OS: macOS
• Browser: Chrome
• Version of the Theme: v1.12.0
• Version of Hugo (run a hugo version if unsure): v0.111.2+extended

Additional context
I changed this filter to plainify, and it's works. But I don't know why it works.
https://github.com/lxndrblz/anatole/blob/master/layouts/index.html#L56

- <p>{{ .Summary | markdownify }}</p>
+ <p>{{ .Summary | plainify }}</p>