I do not know of a study that has looked specifically at baby carriers (as in a peer reviewed academic study). I imagine you may find something on the use of things like jumparoos and why their use should be limited - the same thinking would apply to something like the bjorn in terms of positioning.
I realize it's not an academic study, but this site has an explanation of proper positioning in a carrier: https://www.storchenwiege.com/babycarrierresearch.htm.
There's more here: https://www.babywearingschool.com/drkirkilionis.htm
Apparently Dr. Eveline Kirkilionis (who is German I believe) has written about this a fair amount but maybe only in German as I only see her referenced.
In any case, it makes sense that a carrier should mimic the way a baby would be held in arms - the bjorn doesn't do that.
It's pretty easy to feel how the "crotch dangle" stresses the spine (stolen from the Didymos instructions) - stand on one leg and hold your other leg out in front of you bent at the knee. Clasp your hands under your knee - you'll feel supported. Now clasp you hands right under your bum - you'll feel stress on your back. To me, that's a pretty clear indicator of what happens to a baby in something like the bjorn. Is that bad is used sparingly - probably not. Would I want to wear kiddo like that for hours a day - nope!
Re: **epphd** re positioning in carriers
So I did finally find the Kirkilionis articles through WorldCat - they are in German though and mine is too rusty to translate ;-)
But... (sorry for the formatting)
Tragen eines Saeuglings Prophylaxe bei angeborener Hueftdysplasie E. Kirkilionis
KRANKENGYMNASTIK, 50, no. 3, (1998): 411-421
My guess is that this subject has been more discussed medically in Germany (as that's where modern babywearing really got going) or elsewhere in Europe than in the US.
And this through German Wikipedia...
Evelin Kirkilionis: Ein Baby will getragen sein. Alles ?ber geeignete Tragehilfen und die Vorteile des Tragens ISBN 978-3466344086
Breastfeeding Counselor with Breastfeeding USA
Babywearing Guide ** Newborn Carriers
Cloth Diaper Guide
Safe Bed Sharing Info