Hey, this is a known issue. Inline preview uses a WYSIWYG engine that re-serializes the markdown when you switch modes or tabs. Because of that, --- frontmatter separators get interpreted as horizontal rules, and ** in glob patterns gets interpreted as bold formatting. The content then gets written back to disk with no warning.
Workaround: use the classic side-pane preview with Cmd+Shift+V instead of the in-place toggle. This won’t affect the file contents.
More details in this thread, it describes the same issue: Bug Report: Preview Mode Strips Content and Formatting When Switching Back to Markdown Mode
The team is aware and your report helps with prioritization. Let me know if there’s anything else.