Direct rollover of designated Roth to Roth IRA is excepted from income inclusion pursuant to §408A(d)(3)(B).  Rollover from 401(k) and 60-day rollover, on the other hand, will still be taxed for distributions that do not represent return of basis.

I think the problem is that it is not uncommon for the tax treatment of backdoor Roth IRA, in terms of the rules on prorata income inclusion and IRA aggregation, to be misunderstood.  When the 1099-R has the "Taxable amount not determined" box checked, it is easy to presume that the taxpayer can elect to convert only previously nondeductible contributions, when they really cannot.