Safari sort not the same as finder
I haven't tested thoroughly yet, but the problem seems to happen for me when I leave Safari (11.1.2) open overnight. Quitting Safari and restarting it fixed the problem when I tried it today.

A similar bug has also been reported in Chrome ( https://bugs.chromium.org/p/chromium/issues/detail?id=392754 ).  In that case restarting Chrome fixed the problem.

It feels to me as though the code providing the document picker is caching today's date for some reason and using that to compare the "Date Added" metadata to.  So it's more of an MacOS bug than a Safari bug.

Indeed if you look at the end of the thread here: https://discussions.apple.com/thread/3272875?start=45&amp;tstart=0 there are several applications that show the same problem when left open overnight.