I've dealt with similar situations and believe all is not lost. I would speak with an IRS representative and receive an impression that all is well and then discover these letters being sent to the client requesting money. I cannot remember a situation where I was not successful in getting those fee assessments abated. I'm convinced that it's a simple GIGO equation with the computers at the IRS. When the representative fails to key in the case correctly, the computers go to auto-pilot in mailing out the letters (which effectively forces you to contact that IRS). This is unlike collection agencies where they continuously contact you. So, yes, I is often an inconvenience to engage with them again. But, that's usually expected until you get a full resolution of whatever is going on. I've learned to expect some of those 'unexpected' surprises over the years
Good Luck!