Box 1, 2a on 1099-R have different amounts, but TT still wants to copute box 2a

My 1099-R has boxes 1 (gross distribution) and 2a (Taxable amount) both filled in and nothing checked in box 2b.  TT still wants to compute box 2a.  In the past, TT has just accepted amount filled in box 2a by Payer.  Why has this changed???