First and foremost, the classic recipes states quite clearly, "Dough is ready when its surface is dotted with bubbles." Expecting microorganisms to do their work in precisely the same amount of time, every time, requires a good bit of precision on your part.
Why the slow rise? Almost definitely a yeast problem. The question is, which kind of yeast problem? Acid can kill yeast, heat can kill yeast, salt can kill yeast, sitting in a package forever can kill them, and there are other, less likely scenarios too.
Are you using jarred active dry yeast or individual packets? You mention testing it, but what technique did you use, a little water and sugar, watch for bubbles? You can (and probably should) do this at the beginning of the recipe, so that your yeast is kickstarted into alertness.
Stainless steel is not the problem. It's non-reactive.
Vinegar is likely to slow the growth, so you probably just need to give it more time. The best way to judge readiness is by the dough, not by the clock.
Another factor is that a few degrees difference can make a difference in the rate of yeast growth and CO2 production:
"With glucose as growth-limiting substrate in the chemostat and aerobic conditions in the respirometer, the carbon dioxide output was found to be higher as temperature at which the organisms were cultured increased from 25 to 39" --Journal of Microbiosology
Hope these help, but I think there's not just one answer here.
Your picture makes me want to say that it's a classic case of underproofing, but... You also mixed that dough at a pretty high speed. I think you may have over-developed the gluten. Sometimes if the gluten is too tight, it won't be stretchy enough to accommodate proofing and oven spring, which can lead to a rough surface during proofing and ripping while baking. This is more frequently a problem for buns since their shape requires them to be molded tighter than a loaf would normally be, and their small size means that they rise faster than the gluten can relax. You might have better luck only mixing for the recommended time on the lower speed, and then folding it once or twice during the rising time.
Best Answer
"Cloud buns" and your bread dough are really two different products. The dough in the video is likely based on a "milk bread" recipe, which results in the smooth texture that is pictured. It reminds me of the Momofuku Pork Bun, a recipe I have made often. This recipe produces a very smooth dough, which is portioned, shaped, and steamed. It has a much different texture than my rustic sourdough loaves, which are generally a higher hydration than the dough you've pictured, but not nearly as shiny and smooth as the milk bread. It's just the nature of the recipe and the ingredients. You can knead all you want, but you are not going to get the same texture and sheen, likely because your bread dough does not include milk and fat.