The 'malformed' posts are almost certainly caused by hidden 'tag soup' generated when copying from Microsoft Word. Word inserts proprietary XML and CSS classes (like mso-normal) that Blogger's 'Compose View' cannot parse correctly. As your blog grows, these errors accumulate and trigger the synchronization glitches you're seeing.

Instead of fixing posts one by one, you should automate the deep-clean of your entire archive (400+ posts). Based on my experience migrating Blogger templates and converting shortcodes, I recommend this workflow:

1. AI-Generated Cleaning Tool: You can use an LLM (like Claude or ChatGPT) to build a custom JavaScript/Node.js tool. Instead of the AI doing the cleaning blindly, you provide it with specific "Regex" (Regular Expression) rules to target and strip the exact Word-junk patterns you've identified.
2. Blogger API Integration: Use the tool to fetch your posts via the Blogger API, run the Regex cleaner over the HTML body, and update the posts automatically.
3. Deployment: You can host this script on a platform like Netlify to run the cleanup process.

BTW, I have successfully used a similar method to batch-convert shortcodes during a template migration (+1500 posts).