All lines are rendered as header in edit mode, not just the header line

**Describe the bug**
A clear and concise description of what the bug is.

**To Reproduce**
Steps to reproduce the behavior:
1. Create a multiline block starting with a header marker

**Expected behavior**
Only the header is rendered big when in edit mode

**Screenshots**


**Desktop (please complete the following information):**
- OS: Linux Mint 20.1
- Browser desktop client
- Version 0.1.0

Reproduce the issue in the desktop client using version 0.1.0 on Linux Mint 20.1: create a multiline block starting with a header marker and inspect edit mode. Trace the editor's rendering path for that block, then confirm that only the header line is rendered large while the remaining lines retain normal formatting.

Written by the indexing model from the issue text.