I'm in a Bootstrap 3.3.7 environment (no, I can't go to BS4 here unfortunately) and when opening a modal from a modal, the new child modal will not scroll.
I've tried other fixes such as assigning a height and overflow but the problem then is the modal is at a set height and will not scale with the window size so any responsiveness is thrown out the window.
Was able to make the scroll wheel control the scrolling in the child modal, but that isn't the greatest UX to use (and I had to hide the new scrollbar so there wasn't any duality there to cause confusion).
Bootstrap modal inside a modal; Cannot scroll after closing modal looked promising and addresses the issue of modal-open no longer being in the body tag. The problem is the script does not seem to work because modal-open is not being placed in the body tag when the child modal opens.