I'm so sorry.
Your story brought back some painful memories. Yes, I have had a very similar experience.
Years ago, I married into a home with a semi-feral dog. Eddie wasn't unfriendly, just untrained and fairly aloof by nature. I took him and me to obedience school, got him neutered, and spent a lot of time with him. We all became happier quite quickly. There were things about his personality that were simply baked into the cake that made him perhaps not the ideal pet, but he was much easier to have around.
He was also, shall we say, due for some deferred maintenance. His teeth really needed cleaning, and given his age of only about 6 years, this wasn't something we could put off and think he'd die first. He was a young and extremely healthy dog whose siblings were active sled dogs.
The morning of the teeth cleaning appointment, I had some misgivings. I thought I was just being... I don't know, weird about it? There was no reason for these feelings. They weren't rational. This was the best veterinary clinic in the county, and my best friend was the vet!
I took Eddie to the appointment, with the same sense of dread the whole way. Still, I dropped him off. And whether he really liked it or not, I gave him a kiss on the head. I turned my back and knew I'd never see him again.
An hour and a half later, the vet called. Before she could say a word, I said "I know." She talked to my spouse who made everything worse by threatening legal action and everything else under the sun. Spouse would not allow an autopsy, which the vet (I mentioned the vet was my best friend, right?) really wanted to have done. She understood that she might not be the one to do it, but how a young healthy dog could just drop dead in the recovery room was beyond her comprehension. There simply had to be more to this story.
Years later, we found out that there was. My friend was still haunted by this death, and ran across an article in a journal about sled dog sudden death syndrome. Young healthy dog has a good workout or some minor stress and then just dies. It was an electrical issue that had inadvertently been bred into these dogs. That's undoubtedly what happened to Eddie. Although it perhaps could have happened at any time, the additional stress of anesthetic was the likely suspect.
Everybody who's reading this is probably asking why I took him to the appointment since I knew what was going to happen. I don't have a good answer for that. I can blather all I want about my fears being irrational and based on nothing tangible, but they did come true. We talk ourself into stuff, right? It's what we do. We shelve the mysteries, the unexplainable, the gut feelings, the hunches, and we tell ourselves that we're rational human beings and are not subject to being spooked like that. And I have another answer too, that's perhaps even less satisfactory. Eddie's time was up, and somehow, some way, it was my job to take him to that appointment. It was my part in that fraction of the cosmic play. I did it under a significant weight. Somehow, in a way I will never be able to understand or explain, it was the right thing for me to do at that moment.
It's what gave me calmness then and it's what gives me peace now. I'm not saying that it was fine, or all right, or good, because it wasn't. It sucked. It still sucks, even though had he lived a full life he'd still be long dead by now.
I hope in some way this story connects with you. Feel free to PM if that would help, and know that I'm sending you good vibes.