For a few years now, according to the sharing image site Wikimedia Commons, the messy orange squiggle on the left is the genuine example of a  "David Bowie signature" . 

The curators of the "Bowie Is" exhibition believed them because they reproduced it under the quote on the wall to the entrance of the brilliant exhibition at the V&A and thereafter globally...thankfully some real examples (loaned by Bowie) were on display inside.

https://commons.wikimedia.org/wiki/File:David_Bowie_signature.svg

For the offending photo...see below. I wonder what Bowie thought when he visited?