The IRS sent me a letter that they adjusted my taxes and (substantially) reduced my overpayment applied to next year. Are they going to send me an explanation?

@arstep2 - yes, that should arrive in a few weeks.  Most every one who posts that their refund was reduced has to do with the Recovery Rebate Credit / stimulus or the Child Tax Credit.

1) what was the EXACT amount of the IRS adjustment?

2) what is on Line 28 of your tax return (Form 1040); if other than zero, what is on Line 14f of Form 8812?

3) what is on Line 30 of your tax return? 

we can probably figure out what happened.