If your refund increases when you enter your basis in nondeductible traditional IRA contributions, this implies that you have an otherwise fully taxable distribution or Roth conversion being reported on your 2024 tax return. When you add the basis, TurboTax then performs the required calculation on Form 8606 Part I to determine the nontaxable portion of that distribution or Roth conversion. However, you must also make sure that you have entered your 2024 year-end balance in traditional IRAs for the calculation on Form 8606 to be performed correctly, otherwise the result will be an incorrectly high nontaxable amount due to TurboTax incorrectly assuming that your 2024 year-end balance in traditional IRAs was zero.
Regarding the intervening years, no Form 8606 was to be included for a particular year if there were no contributions or distributions that required Form 8606 to be included. In that case, your last filed Form 8606 is still current with respect to your basis that carries forward. If no Forms 8606 were filed for the intervening years, the assumption is that you had no traditional IRA distributions to report for those years otherwise you've filed incorrect tax returns.
Note that TurboTax will silently delete your basis information if you ever inadvertently answer No when asked if you have such basis. MANY people have been caught by this deficiency in TurboTax's programming, but TurboTax Product Quality just claims that this is user error.
