Wow, very observant of you. I just did some testing of previous versions of TurboTax and this bug is been in TurboTax at least as far back as 2013, probably farther. TurboTax is incorrectly omitting the ROLLOVER indication on a rollover of a qualified retirement plan to a Roth IRA (or an In-plan Roth Rollover) if the taxable amount is $0.
The only workaround I've been able to come up with is to use the CD/download version of TurboTax, switch to forms mode, open the code G 1099-R form, unmark box B5 that is presently indicating that the entire distribution was rolled over and instead fill in line B6 with the gross amount of the distribution. However, this messes up TurboTax's tracking of your 2020 conversion contributions to your Roth IRAs and messes of TurboTax's 1099-R Summary, which could have negative effects elsewhere on your tax return if you also took Roth IRA distributions in 2020.
My only other suggestion would be to print and mail your tax return after hand-writing in the ROLLOVER indication. Printing and mailing would also be required if you were able to do an override to correct this (but, as you found, you can't). I really don't like the idea of printing and mailing the tax return, though, because you are relying on the accuracy of the data transcriber for your IRS-recorded tax return to be correct.
I suspect that the best approach might be to ignore the error. You and the IRS will be getting a Form 5498 showing the rollover contribution to the Roth IRA and the code G on the Form 1099-R itself shows that there was indeed a rollover.
@macuser_22 might be able to pass this bug report along to the moderators. I suspect that since the bug has been around for so long and hasn't seemed to have caused any problems with the IRS for the affected taxpayers, it might not get much attention.
Note that the problem also seems to happen with an indirect rollover from a qualified retirement plan to a Roth IRA (code 1 or 7) when the taxable amount is $0, but I've only checked some of those variations.
