I'm here to provide some insights about the missing expense transaction in reconciliation in QuickBooks Online (QBO).
For clarification, is an All expenses transaction? If so, you'll need to check the register if what is the status of that transaction. And if it's recorded and it doesn't appear as R, ensure to that the filter is All since there's a tendency that you're only viewing the deposits tab.
If the issue persists after all the steps above, let's try logging in your QBO account in a private browser or an incognito since there's a possibility of a cache issue.