Never had that option checked. Also been my preference to open manually. I admit, I did chuckle about this because your usual reply is “you can’t replicate” because you were on Windows. Now I knew you were Mac and could try.
Firefox did work. Because of the nature of Reunion, import works but isn’t pleasing. There’s a demo o GedItCom II but only for 15 days so am waiting for final download I do. Never learned how to use it well but it’s “proper” GEDCOM (Reunion isn’t).
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The fact it works for you in Firefox and not Safari strongly suggests it is a browser issue.
For me it works in all the browsers I have available. (Safari(15.1), Firefox, Chrome). In Safari it works whether the above setting is ticked or not. Just that one saves it as a *.zip and the other as a no-suffix text file (i.e. a ged file without the .ged suffix). The latter, I suspect, is because I didn’t have .ged in the file name set for it in kiwitrees.
So I can only guess that it’s an older problem with your version of Safari.