Why does adding a 1098 from another mortgage bank lower my deductible interest? Is this a bug?

I'm a long-time homeowner. In 2020, I refinanced my home. Due to the refinance, and switching mortgage banks, I made mortgage payments to three different banks in 2020:

  • Bank A (original mortgager): 8 monthly mortgage payments, January - August
  • Bank B (temporary mortgager): 1 mortgage payment, October
  • Bank C (new mortgager): 2 monthly mortgage payments, November - December

I have received 1098 statements from all three mortgage banks. Now I'm doing my 2020 taxes through TurboTax Online.

When I enter the 1098s for Banks A and C only, it shows me this summary:

tax1.png

Notice All Deductible interest is $17,446

After that, I enter the 1098 for Bank B. This is the new summary it shows me:
tax2.png

 

Notice that All Deductible interest has now dropped to $8,908.How is this possible?  Just entering the 1098 for Bank B increased my tax bill by thousands of dollar because it switched me from itemized to standard deduction!

I don't understand how reporting that I made another mortgage payment (of which most was interest) caused my deductible interest to drop precipitously rather than increase.


Is this a bug in TurboTax?