As @argentum commented on discord, this is a feature of mdnotes and it’s described at https://argentinaos.com/zotero-mdnotes/docs/next/advanced/hidden-prefs:
- Setting
extensions.mdnotes.obsidian.blocksto true will append a block ID to all non-header text blocks of exported Zotero notes, i.e. to each highlight and annotation extracted by Zotfile.