I thought I had done this before in Windows, because I put a lot of memories into my H3 before.
No, this has never been a thing. I'm sure it sounds trivial but copy and paste of structured data between applications is not a universal thing. LibreOffice converts the columns into tab-separated fields without any quoting, strips leading zeroes and omits some empty fields. Other applications may do different things, mangle the format in various other ways, and of course, behave differently on other platforms.
What am I doing wrong?
You're trying to do something that has never worked.
I hate that we cannot import a CSV any more, like we used to do.
If you're having trouble importing or opening CSV files then file a bug with a debug log.
File->Import from file works from CSV (or any other file type CHIRP supports). If you want to replace your entire contents of memory with the CSV file, then this will do what you want. It's never (ever) what I want, so I would recommend anyone else just open the edited CSV file in CHIRP and copy/paste (within CHIRP) the memories you want from the updated file.
--Dan