I have been having this same issue. I believe this is more of a limitation of Safari than it is an issue of your creation. The only “tricks” I was able to find online are
overflow-y: scroll; -webkit-overflow-scrolling: touch;
I have added this to my container and child elements with no luck. I even added a pseudo element that was slightly bigger than the container hoping to trigger the scroll-bar on load, but this did not work either.
Lastly, I tried adding scroll-bar styles to the container hoping that would force it, but as you probably can guess: no dice.
I reached out directly to Safari technical support, and I hope to have an updated answer within the next couple of days. If anyone has come up with a working solution, please let me know!