Serious flaw in macOS: Quarantine can stop you from opening documents

“Last week, I looked at quarantine flags on documents, and wondered what their true purpose was. I think I’ve found it: they can stop you from opening documents, even those which are known to be free of malware and have never been near the Internet,” Howard Oakley writes for Eclectic Light Company. “Yes, in macOS the purpose of document quarantine is to get in your way and ruin your day.”
“Find a document, any document, so long as it has a quarantine flag attached to it. This could be because it was downloaded from the Internet, or has merely been opened by a sandboxed app such as Preview,” Oakley writes. “Then make a copy of it: don’t do this with the original, as by the end of this all you’ll want to with that copy is trash it, as it will drive you crazy.”
“Select that copy and use the Finder’s Get Info command to change the app which opens it to something other than the default for that document type. It doesn’t matter which app it is, so long as it’s not the default,” Oakley writes. “Once that’s selected, and without applying this change to other documents of that type, close the Get Info dialog, and double-click on the document icon to open it using that new app.”
“You should see a dialog similar to that above, which refuses to let you open that document apparently because it is from an unidentified developer (not source), and your security preferences won’t allow it,” Oakley writes. “None of this makes any sense, does it?”
“Select that document again, open the Get Info dialog, and set the app to open it back to the default, the first item in the popup menu. Close Get Info, and double-click on the document,” Oakley writes. “It should open fine now. But if you ever again try to change that document’s individual setting for the app to open, you’ll find that macOS once again refuses to open it, with the same strange dialog which makes no sense at all.”
Wht this happens and how to work around/ fix it here.
MacDailyNews Take: Just in case this has happened or is happening to you, now you’ve got the answer key!