In Safari 13 release notes it is stated that there is no longer the need to apply the following to an element to enable the bounce scroll effect:
div { overflow
I think you should try to change that using the overflow property that in Safari blocks the bouncing scroll behaviour. To do that in the parent container of your scrolling div you have to set:
overflow: hidden;
and then in your div set something like this:
div { overflow: auto; }