The path mechanics are set up to choose the valid path closest to the center of the garden (or arbitarily, if 2 are equidistant) so that behavour is intentional, BUT (a) there's no way for you to know that which makes it impossible to optimise for, (b) it probably is more reasonable to expect it to score the longest path.
I'm a bit torn, because I think central is more likely to look pretty, and I've tried to build rules where pretty gardens score well (this hasn't been completely successful, but it is the intent). I'll have a think about it.
Thanks for pointing this out. And for playing; glad you're enjoying it.