Print form 9325 and the client e-file history?

Use the "unique" serial number from your return found on your ACT and Form 9325.  Call the EFile help desk and they will help you research the problem.  I have been forced once to use "snail mail" to provide this data to the Service.

That's scary, because it happened pre-Covid.  Is this another CP80, that says there is a credit there but no return?  And you're sure it's 2018, with the statute about to expire in three months?

I understand how they can lose paper returns but not confirmed efiled ones.

Maybe the taxpayer's last name is Spam and it went into the IRS folder for those.

Or perhaps Null?

https://www.bbc.com/future/article/20160325-the-names-that-break-computer-systems

Years ago, a vanity plate owner in California had a personalized license plate that read "NO PLATE".

He/she got a huge number of delinquent ticket notices. 

True story.