Zotero only syncs explicit deletions, so the only way for things to disappear across devices would be for someone to manually delete them. That could happen if, say, you left Zotero set up on a shared computer and someone with access to that computer cleared out the library.

It looks like you've since recreated things? Maybe by reimporting from a file and using Restore to Online Library? That clears some of the syncing history, so I'm not able to see when any deletions were made.

If you're not sure what happened, I'd recommend going to your key settings and deleting all "Automatic Zotero Client Key" entries except for the most recent one. You may then have to set up syncing again with your username and password on any other computers, but you'll be sure that no one else has access to your library.