I agree it's about the individual horse being confident going out alone, and on my last horse shopping experience that was something at the top of my list- including when I went to test ride, I was only working with sellers where I could take the horse out on the trail and have some of that time be where I would ride the horse away from its buddies. While I've had some nice trail rides on geldings, I've only ever owned mares. Have ridden a few hundred trail miles the last couple of years, the majority solo, all on my mares.
The buddy sour thing though, my oldest mare is desperately buddy sour (she's why I have 3 horses instead of two!). She doesn't even like my youngest mare, but she completely loses her mind when I take the younger one out to ride- even if I'm riding in the field across the street where she can literally see us. She runs the fence, calls incessantly, and coats herself in sweat. I can longer leave her in the field alone, e.g., taking her up to the barn last. Could I work with her on it? Probably? Would she improve? Maybe a little? But she's retired and nearly 26, and 99.9% of her time she isn't alone so it's something I work around. Weirdly, she was never like that when I was riding her a lot of miles alone, and I never once had a problem with her refusing to ride out. I think it's something that's gotten a lot worse since I moved the horses home; she had spent nearly 10 years in larger, busy boarding barns and I think she may have been a bit more comfortable in that kind of environment.
Edited to add: The buddy sour mare I mentioned, she does have notable heats and rather than the typical "moody, bitchy mare" attitude, I always found her to get sort of lethargic and quiet, clearly uncomfortable. I generally gave her a day or two off, or just took an easy walking ride, when she was quiet like that. She also (still) has the tendency to want to show heat signs whenever she meets a new horse. The other two mares I have here at home, couldn't tell you when either of them have ever been in heat as neither have ever shown any discernible signs.