I'm coming from where this was crossposted in r/surveying

I think what you said is generally true however I wanted to tack something on. In most cases of adverse possession the disputed land should be very clearly occupied by the encroaching party and this is something that would most likely be shown on a survey. Whether it's gardening/landscaping, sheds, other permanent structures or a fence as you noted. If it's just empty unused space after what grounds do you have to possess it?

Also in their duties of survey a surveyor really should talk with all parties involved which would also alert them of this but I think this often doesn't happen unless as you said, there was a known boundary dispute.

Again as you said a surveyor would most likely not discover this "issue" depending how obvious this so called shrub boundary is supposed to be, but for that reason it seems like a weak AP claim