The bug was originally reported here in the Obsidian's Admonition-plugin repo, but then became clear that the bug is not in the plugin, nor is it at the CodeMirror end.
Copy of the original bug report (please check for more comments):
How to reproduce the bug
- Add 3 test lines inside the admonition:

- Use Obsidian/markdown quote feature(
>) on the second line. BUG: Notice that syntax highlighter stopped working on all the lines even below the quoted one.

Other interesting detail
Putting a code block returns highlighting back to normal.

Versions
- Obsidian 0.12.10
- Admonition 6.2.2