After Tammi got really sick, a lot of people claimed that Motown used Valerie Simpson to complete (in whole or in part) some product that was released as being by Tammi.
It's a contentious subject, and Valerie denies it (but in such a way that allows for it being a wee bit true).
Google will point you to a lot of fan forums, where the discussion is mostly emo type fancrap For me, the most likely scenario is that Valerie did guide vocals on some tracks, Tammi came back when she could and did what she could. What she couldn't do, Motown used Valerie to finish the job.
For me, that's a reality-based speculative conclusion, and AFAIC no harm no foul, considering the circumstances.