@Joeyfwb - the Advance Child Care credit (LIne 28) and the Recovery Credit (line 30) are separate issues.
the Recovery Credit is in multiples of $1400 so it may be coincidence that the additional refund to you was $1400 or maybe it was related to the Recovery Credit and the wrong letter was sent, but the whole thing does seem "odd".
a few things to check (and please post back)
1) what is on Line 30? since you state that you received the 3rd stimuls last spring, this line should be $-0- please confirm
2) what is on Line 28? since you state you claimed no children, this line should be zero as well.
3) was there a form 8812 completed as part of your tax return? there shouldn't be if there were no children claimed.
4) access your transcript at IRS.GOV and see if you can decipher any clues on why they think they owed you $1400.
