You are correct. A bug in TurboTax is being triggered by indirectly rolling over a after-tax money from a qualified retirement plan to a traditional IRA, a perfectly legitimate transaction (although not particularly wise since the recipient of the distribution should have rolled the after-tax money over to a Roth IRA instead, as you did). Not only does the nontaxable portion (in your case the entire distribution) erroneously get included on the 2019 line of Form 8880, it also gets included twice on line 25 of the 1099-R Summary. TurboTax simply refuses to treat the after-tax money as having been rolled over to a traditional account.
I just checked and this same bug is present in 2018, 2017 and 2013 TurboTax. I didn't check other years, but I wouldn't doubt that the bug has been around since well before 2013.
It's impermissible to indirectly roll after-tax money over to a qualified retirement plan like a 401(k) instead of to an IRA, so maybe that has something to do with how this bug came to be. TurboTax does not ask the type of traditional destination account, so it can't distinguish between permissible and impermissible rollovers of after-tax money to traditional accounts.
Note that since no tax should be withheld on a nontaxable distribution, it's easy to work around this bug by simply entering code G instead of code 1. With no tax withholding the IRS will be unaware that this substitution was made to obtain the correct result.
I can understand why you initially reported is as rolled over since that's technically what you did. The term "conversion" is used by the tax code and the IRS only with respect to distributions from traditional IRAs, not for distributions from qualified retirement plans, but TurboTax uses the term "conversion" for any movement of funds from a traditional account to a Roth IRA.
I'll see what I can do to about reporting this bug.
