It does not happen in troubleshooting mode. Moreover, in regular mode, it only happens in a group library, not in my main library. So what should I do next?
If it doesn't happen with all plugins disabled, you'll need to identify the plugin that's causing it and then report the problem to the plugin's developer.
What are the exact steps you're taking to trigger this? Can you make a screen recording and email it or send a link to support@zotero.org with a link to this thread?
If so, can you provide a Debug ID for reproducing it?
(By the way, I was wrong about this not happening in my main library; it happens there too.)