IRS "Made changes" notice

A client received a letter that I have never seen.  The notice tells them that the IRS has made changes to the form 5500 and as a result the client owes $49,500 + interest (see attached).  What is this?

Did the IRS say what changes were made? The CP220 (at least as I understand it) is supposed to show what changes were made.

Out of curiosity, how can IRS make changes to the 5500 form without informing the plan sponsor unless it was a plan #?

I've never seen this before either.  Was the form perhaps filed late?  And this is for late filing penalties?   Or incomplete (i.e., missing schedules or attachments that relate back to IRS (as opposed to DOL) portions of the form)?   How would IRS even know what to "change" on an "information" return?   I would contact the phone number on the notice and ask what's up.

https://benefitslink.com/boards/topic/55013-cp-220-notice/

Here's a link to prior discussions on this topic.