Yes, it is just mis-segmentation, no doubt. Again, you sometimes see the same thing in published inscriptions, e.g., *chandogasa-brahmacārin for chandoga-sabrahmacārin.
I was surprised to see that in the lists of gotras circulated on the web, in many cases, the "sa" of "X-sagotra" has been attached to the name of the gotra itself, i.e. "Xsa". One can see several examples in the list below:
http://familypedia.wikia.com/wiki/Brahmin_gotra_system
This list includes "Ātreyasa", for instance, as a gotra-name. The only way I can think how this came about is a misanalysis of "Ātreya-sagotra" as "Ātreyasa-gotra." I have not checked the names listed in texts like the Gotra-pravara-mañjarī to see if
this phenomenon has affected the lists in Sanskrit texts as well.
Madhav Deshpande
Ann Arbor, Michigan, USA