I ran into similar issues the past few days.
As far as I can tell this happens when your modals are nested in the React tree, i.e.

<Modal>
  {showInnerModal && <InnerModal />}
</Modal>

A tab event on the inner modal will be bubbled up by react to the outer modal as well, and cause a scopeTab() to be triggered twice, first inner modal, then outer modal. Subsequently, the "safari radio button workaround" code (here) then causes the outer modal to be focused in Safari, but not in other browsers.

If you can structure your modals to not be nested I think this issue does not occur. However, this is not always feasible (nor is it for us):

<>
  <Modal />
  {showInnerModal && <InnerModal />}
</>

I think putting an event.stopPropagation() in the handleKeyDown here should fix it, but even then focus in Safari does not seem to be working reliably, but I still have to dig into it a little further (debugging focus code in Safari is a bit... erhh...). One thing worth noting is that the focusManager's modalElement is always undefined due to being passed an undefined this.node in the open() handler, here. Not sure if this has anything to do with the other broken focus behavior I'm noticing. Perhaps someone more familiar with the codebase can chime in.

The fix mentioned by @pleunv worked for me and I've opened a PR to prevent propagation on that event.

I was noticing other focus issues in Safari such as the nested modal's content div losing focus after the modal opens, but it seems to be a separate issue that I haven't managed to track down yet.