It may have to do with the 2021 showing Line 19 (1040) as a "non-refundable" (which changes the math for line 21 for M1PR worksheet) vs in 2020 Line 19 (1040) was simply the "Child Tax Credit" (not specifying refundable vs non-refundable).
Refundable vs. Non-Refundable Tax Credits | IRS.com
For me, this slight change meant Line 21 in 2021 is $0 whereas Line 21 in 2020 for me was $6000.
Bummer, I do hope I am incorrect, I was surprised getting a big ol' $0 for a Property Tax Refund. Possible that overall I come out ahead with additional "Child Tax Credit" money offsetting the loss of the reliable Property Tax Refund.
