My Megan liked everyone, everywhere, except for the vet's office. Try as we might, we were never able to convince her that it was a good place. She would spit the treats they gave her right back at them. I am the "tech" for all my dogs and it made no difference. She always had to be muzzled for any procedure besides shots. What the heck, when I was a small child, they probably wished they had a muzzle for me at the doctors.
Never in her too short life did she even growl at any person or animal anywhere else. She even schmoozed with the clients while waiting to see the vet. I would not worry about what one vet tech said about Toggle unless she also had a degree in dog psychology. Otherwise it's just an unsolicited opinion, not a diagnosis. I'd just keep trying with "happy" visits where all that happens is that she gets yummies and invest in a soft muzzle to take with you in case it's needed. I've never thought it was a good idea to use a muzzle that has been used on other dogs, particularly at the vet's office.