When your husband's employer-provided retirement plan was a 401(k), the balance in that account is not includible on Form 8606 line 6 when calculating the taxable amount of the traditional IRA distribution. Once that 401(k) balance was rolled over to a traditional IRA it became includible on Form 8606 line 6, making the traditional IRA distribution more taxable and causing more of the basis in nondeductible traditional IRA contributions to remain with your husband's traditional IRAs to be applied to future traditional IRA distributions.
Because that rollover occurred in 2021, that rollover would have similarly affected the taxable amount of any 2021 traditional IRA distributions, but not any traditional IRA distributions before 2021.
